Unknown :format param should result in empty request.formats

actionpack/lib/action_dispatch/http/request.rb

@@ -167,7 +167,7 @@ module ActionDispatch
 
       @env["action_dispatch.request.formats"] ||=
         if parameters[:format]
-          [Mime[parameters[:format]]]
+          Array.wrap(Mime[parameters[:format]])
         elsif xhr? || (accept && !accept.include?(?,))
           accepts
         else

actionpack/test/dispatch/request_test.rb

@@ -432,6 +432,10 @@ class RequestTest < ActiveSupport::TestCase
     request = stub_request
     request.expects(:parameters).at_least_once.returns({ :format => :txt })
     assert_equal with_set(Mime::TEXT), request.formats
+
+    request = stub_request
+    request.expects(:parameters).at_least_once.returns({ :format => :unknown })
+    assert request.formats.empty?
   end
 
   test "negotiate_mime" do