Duplicate possible frozen string from route

Ruby 1.9 freezes Hash string keys by default so where a route is
defined like this:

  get 'search' => 'search'

then the Mapper will derive the action from the key. This blows up
later when the action is added to the parameters hash and the
encoding is forced.

Closes #3429

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -1403,9 +1403,10 @@ module ActionDispatch
 
         def add_route(action, options) # :nodoc:
           path = path_for_action(action, options.delete(:path))
+          action = action.to_s.dup
 
-          if action.to_s =~ /^[\w\/]+$/
-            options[:action] ||= action unless action.to_s.include?("/")
+          if action =~ /^[\w\/]+$/
+            options[:action] ||= action unless action.include?("/")
           else
             action = nil
           end

actionpack/test/dispatch/routing_test.rb

@@ -2678,6 +2678,15 @@ class TestRoutingMapper < ActionDispatch::IntegrationTest
     assert_equal '0c0c0b68-d24b-11e1-a861-001ff3fffe6f', @request.params[:download]
   end
 
+  def test_action_from_path_is_not_frozen
+    draw do
+      get 'search' => 'search'
+    end
+
+    get '/search'
+    assert !@request.params[:action].frozen?
+  end
+
 private
 
   def draw(&block)