add Mime.fetch so we can have default mime types

actionpack/lib/action_controller/test_case.rb

@@ -581,8 +581,7 @@ def build_request_uri(action, parameters)
 
       def html_format?(parameters)
         return true unless parameters.is_a?(Hash)
-        format = Mime[parameters[:format]]
-        format.nil? || format.html?
+        Mime.fetch(parameters[:format]) { Mime['html'] }.html?
       end
     end
 

actionpack/lib/action_dispatch/http/mime_type.rb

@@ -29,6 +29,11 @@ def self.[](type)
     Type.lookup_by_extension(type.to_s)
   end
 
+  def self.fetch(type)
+    return type if type.is_a?(Type)
+    EXTENSION_LOOKUP.fetch(type.to_s) { |k| yield k }
+  end
+
   # Encapsulates the notion of a mime type. Can be used at render time, for example, with:
   #
   #   class PostsController < ActionController::Base