diff --git a/actionpack/lib/action_dispatch/http/cache.rb b/actionpack/lib/action_dispatch/http/cache.rb
index aaed0d750f798768d69885482484d98fecc93393..bea62b94d23a440ce9da546ac5c10f984ea42779 100644
--- a/actionpack/lib/action_dispatch/http/cache.rb
+++ b/actionpack/lib/action_dispatch/http/cache.rb
@@ -4,14 +4,18 @@ module ActionDispatch
   module Http
     module Cache
       module Request
+
+        HTTP_IF_MODIFIED_SINCE = 'HTTP_IF_MODIFIED_SINCE'.freeze
+        HTTP_IF_NONE_MATCH     = 'HTTP_IF_NONE_MATCH'.freeze
+
         def if_modified_since
-          if since = env['HTTP_IF_MODIFIED_SINCE']
+          if since = env[HTTP_IF_MODIFIED_SINCE]
             Time.rfc2822(since) rescue nil
           end
         end
 
         def if_none_match
-          env['HTTP_IF_NONE_MATCH']
+          env[HTTP_IF_NONE_MATCH]
         end
 
         def not_modified?(modified_at)
@@ -43,31 +47,35 @@ module Response
         alias :etag? :etag
 
         def last_modified
-          if last = headers['Last-Modified']
+          if last = headers[LAST_MODIFIED]
             Time.httpdate(last)
           end
         end
 
         def last_modified?
-          headers.include?('Last-Modified')
+          headers.include?(LAST_MODIFIED)
         end
 
         def last_modified=(utc_time)
-          headers['Last-Modified'] = utc_time.httpdate
+          headers[LAST_MODIFIED] = utc_time.httpdate
         end
 
         def etag=(etag)
           key = ActiveSupport::Cache.expand_cache_key(etag)
-          @etag = self["ETag"] = %("#{Digest::MD5.hexdigest(key)}")
+          @etag = self[ETAG] = %("#{Digest::MD5.hexdigest(key)}")
         end
 
       private
 
+        LAST_MODIFIED = "Last-Modified".freeze
+        ETAG          = "ETag".freeze
+        CACHE_CONTROL = "Cache-Control".freeze
+
         def prepare_cache_control!
           @cache_control = {}
-          @etag = self["ETag"]
+          @etag = self[ETAG]
 
-          if cache_control = self["Cache-Control"]
+          if cache_control = self[CACHE_CONTROL]
             cache_control.split(/,\s*/).each do |segment|
               first, last = segment.split("=")
               @cache_control[first.to_sym] = last || true
@@ -81,28 +89,32 @@ def handle_conditional_get!
           end
         end
 
-        DEFAULT_CACHE_CONTROL = "max-age=0, private, must-revalidate"
+        DEFAULT_CACHE_CONTROL = "max-age=0, private, must-revalidate".freeze
+        NO_CACHE              = "no-cache".freeze
+        PUBLIC                = "public".freeze
+        PRIVATE               = "private".freeze
+        MUST_REVALIDATE       = "must-revalidate".freeze
 
         def set_conditional_cache_control!
-          return if self["Cache-Control"].present?
+          return if self[CACHE_CONTROL].present?
 
           control = @cache_control
 
           if control.empty?
-            headers["Cache-Control"] = DEFAULT_CACHE_CONTROL
+            headers[CACHE_CONTROL] = DEFAULT_CACHE_CONTROL
           elsif control[:no_cache]
-            headers["Cache-Control"] = "no-cache"
+            headers[CACHE_CONTROL] = NO_CACHE
           else
             extras  = control[:extras]
             max_age = control[:max_age]
 
             options = []
             options << "max-age=#{max_age.to_i}" if max_age
-            options << (control[:public] ? "public" : "private")
-            options << "must-revalidate" if control[:must_revalidate]
+            options << (control[:public] ? PUBLIC : PRIVATE)
+            options << MUST_REVALIDATE if control[:must_revalidate]
             options.concat(extras) if extras
 
-            headers["Cache-Control"] = options.join(", ")
+            headers[CACHE_CONTROL] = options.join(", ")
           end
         end
       end
diff --git a/actionpack/lib/action_dispatch/http/parameter_filter.rb b/actionpack/lib/action_dispatch/http/parameter_filter.rb
index 1480e8f77c4d30d820a648f8c13075f78b6777e5..490b46c990f9825053b4b4c39a948b5711c49c12 100644
--- a/actionpack/lib/action_dispatch/http/parameter_filter.rb
+++ b/actionpack/lib/action_dispatch/http/parameter_filter.rb
@@ -20,6 +20,8 @@ def enabled?
         @filters.present?
       end
 
+      FILTERED = '[FILTERED]'.freeze
+
       def compiled_filter
         @compiled_filter ||= begin
           regexps, blocks = compile_filter
@@ -29,7 +31,7 @@ def compiled_filter
 
             original_params.each do |key, value|
               if regexps.find { |r| key =~ r }
-                value = '[FILTERED]'
+                value = FILTERED
               elsif value.is_a?(Hash)
                 value = filter(value)
               elsif value.is_a?(Array)
diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index a1ece87d2115d9aae1c92cafc62e6e7a48a53f57..fc840668d50b201e464ed816aa9afeb59b4fe346 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -53,8 +53,10 @@ class Response
     # information.
     attr_accessor :charset, :content_type
 
-    CONTENT_TYPE = "Content-Type"
-
+    CONTENT_TYPE = "Content-Type".freeze
+    SET_COOKIE   = "Set-Cookie".freeze
+    LOCATION     = "Location".freeze
+ 
     cattr_accessor(:default_charset) { "utf-8" }
 
     include Rack::Response::Helpers
@@ -66,7 +68,7 @@ def initialize(status = 200, header = {}, body = [])
       @sending_file = false
       @blank = false
 
-      if content_type = self["Content-Type"]
+      if content_type = self[CONTENT_TYPE]
         type, charset = content_type.split(/;\s*charset=/)
         @content_type = Mime::Type.lookup(type)
         @charset = charset || self.class.default_charset
@@ -142,12 +144,12 @@ def delete_cookie(key, value={})
     end
 
     def location
-      headers['Location']
+      headers[LOCATION]
     end
     alias_method :redirect_url, :location
 
     def location=(url)
-      headers['Location'] = url
+      headers[LOCATION] = url
     end
 
     def close
@@ -158,10 +160,10 @@ def to_a
       assign_default_content_type_and_charset!
       handle_conditional_get!
 
-      @header["Set-Cookie"] = @header["Set-Cookie"].join("\n") if @header["Set-Cookie"].respond_to?(:join)
+      @header[SET_COOKIE] = @header[SET_COOKIE].join("\n") if @header[SET_COOKIE].respond_to?(:join)
 
       if [204, 304].include?(@status)
-        @header.delete "Content-Type"
+        @header.delete CONTENT_TYPE
         [@status, @header, []]
       else
         [@status, @header, self]
@@ -175,7 +177,7 @@ def to_a
     #   assert_equal 'AuthorOfNewPage', r.cookies['author']
     def cookies
       cookies = {}
-      if header = self["Set-Cookie"]
+      if header = self[SET_COOKIE]
         header = header.split("\n") if header.respond_to?(:to_str)
         header.each do |cookie|
           if pair = cookie.split(';').first