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

@@ -1288,9 +1288,10 @@ def decomposed_match(path, options) # :nodoc:
 
         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

@@ -515,6 +515,8 @@ def self.call(params, request)
         match '/sculptors', :to => 'italians#sculptors'
         match '/painters/:painter', :to => 'italians#painters', :constraints => {:painter => /michelangelo/}
       end
+
+      get 'search' => 'search'
     end
   end
 
@@ -2477,6 +2479,11 @@ def test_root_in_deeply_nested_scope
     assert_equal "/posts/1/admin", post_admin_root_path(:post_id => '1')
   end
 
+  def test_action_from_path_is_not_frozen
+    get '/search'
+    assert !@request.params[:action].frozen?
+  end
+
 private
   def with_test_routes
     yield