Reduce TextTemplate cost for simple cases

actionpack/lib/action_dispatch/http/mime_type.rb

@@ -24,6 +24,7 @@ module Mime
   LOOKUP           = Hash.new { |h, k| h[k] = Type.new(k) unless k.blank? }
 
   def self.[](type)
+    return type if type.is_a?(Type)
     Type.lookup_by_extension(type.to_s)
   end
 

actionpack/lib/action_view/template/text.rb

@@ -1,6 +1,8 @@
 module ActionView #:nodoc:
   class TextTemplate < String #:nodoc:
-    def initialize(string, content_type = Mime[:html])
+    HTML = Mime[:html]
+
+    def initialize(string, content_type = HTML)
       super(string.to_s)
       @content_type = Mime[content_type] || content_type
     end