Add propery docs to ActionDispatch::Response [ci skip]

After some discussion on Twitter with @skud, the documentation on ActionDispatch::Response is
a bit sparse. This class is useful when you're writing tests, as often you want to assert various
things about the response that's coming back. Better docs would make this easier for people new
to testing in Rails.

I only added some descriptions for various properties that were defined, and mostly just a sentence
or two. Most of these things are familliar if you're working with HTTP, but some words is better
than no words at all.

Hopefully further commits will fix up things that aren't just documentation.

actionpack/lib/action_dispatch/http/response.rb

@@ -31,10 +31,17 @@ module ActionDispatch # :nodoc:
   #    end
   #  end
   class Response
-    attr_accessor :request, :header
+    # The request that the response is responding to.
+    attr_accessor :request
+
+    # The HTTP status code.
     attr_reader :status
+
     attr_writer :sending_file
 
+    # Get and set headers for this response.
+    attr_accessor :header
+    
     alias_method :headers=, :header=
     alias_method :headers,  :header
 
@@ -49,9 +56,12 @@ class Response
     # If a character set has been defined for this response (see charset=) then
     # the character set information will also be included in the content type
     # information.
-    attr_accessor :charset
     attr_reader   :content_type
 
+    # The charset of the response. HTML wants to know the encoding of the
+    # content you're giving them, so we need to send that along.
+    attr_accessor :charset
+
     CONTENT_TYPE = "Content-Type".freeze
     SET_COOKIE   = "Set-Cookie".freeze
     LOCATION     = "Location".freeze
@@ -93,6 +103,7 @@ def closed?
       end
     end
 
+    # The underlying body, as a streamable object.
     attr_reader :stream
 
     def initialize(status = 200, header = {}, body = [])
@@ -142,6 +153,7 @@ def status=(status)
       @status = Rack::Utils.status_code(status)
     end
 
+    # Sets the HTTP content type.
     def content_type=(content_type)
       @content_type = content_type.to_s
     end
@@ -216,11 +228,13 @@ def delete_cookie(key, value={})
       ::Rack::Utils.delete_cookie_header!(header, key, value)
     end
 
+    # The location header we'll be responding with.
     def location
       headers[LOCATION]
     end
     alias_method :redirect_url, :location
 
+    # Sets the location header we'll be responding with.
     def location=(url)
       headers[LOCATION] = url
     end
@@ -229,11 +243,13 @@ def close
       stream.close if stream.respond_to?(:close)
     end
 
+    # Turns the Response into a Rack-compatible array of the status, headers,
+    # and body.
     def to_a
       rack_response @status, @header.to_hash
     end
     alias prepare! to_a
-    alias to_ary   to_a # For implicit splat on 1.9.2
+    alias to_ary   to_a
 
     # Returns the response cookies, converted to a Hash of (name => value) pairs
     #