use accessors on the request object for manipulating env

this reduces the API footprint for the env hash so that we can be more
flexible when changing API in the future

actionpack/lib/action_dispatch/http/headers.rb

@@ -41,16 +41,16 @@ def initialize(request) # :nodoc:
 
       # Returns the value for the given key mapped to @env.
       def [](key)
-        env[env_name(key)]
+        @req.get_header env_name(key)
       end
 
       # Sets the given value for the key mapped to @env.
       def []=(key, value)
-        env[env_name(key)] = value
+        @req.set_header env_name(key), value
       end
 
       def key?(key)
-        env.key? env_name(key)
+        @req.has_header? env_name(key)
       end
       alias :include? :key?