drop runtime conditionals in parameter parsing

If we only deal with proc objects, then we can eliminate type checking
in the parameter parsing middleware

Author: tenderlove

actionpack/lib/action_dispatch/middleware/params_parser.rb

@@ -13,7 +13,13 @@ def initialize(message, original_exception)
       end
     end
 
-    DEFAULT_PARSERS = { Mime::JSON => :json }
+    DEFAULT_PARSERS = {
+      Mime::JSON => lambda { |raw_post|
+        data = ActiveSupport::JSON.decode(raw_post)
+        data = {:_json => data} unless data.is_a?(Hash)
+        Request::Utils.deep_munge(data).with_indifferent_access
+      }
+    }
 
     def initialize(app, parsers = {})
       @app, @parsers = app, DEFAULT_PARSERS.merge(parsers)
@@ -33,20 +39,10 @@ def parse_formatted_parameters(env)
 
         return false if request.content_length.zero?
 
-        strategy = @parsers[request.content_mime_type]
+        strategy = @parsers.fetch(request.content_mime_type) { return false }
 
-        return false unless strategy
+        strategy.call(request.raw_post)
 
-        case strategy
-        when Proc
-          strategy.call(request.raw_post)
-        when :json
-          data = ActiveSupport::JSON.decode(request.raw_post)
-          data = {:_json => data} unless data.is_a?(Hash)
-          Request::Utils.deep_munge(data).with_indifferent_access
-        else
-          false
-        end
       rescue => e # JSON or Ruby code block errors
         logger(env).debug "Error occurred while parsing request parameters.\nContents:\n\n#{request.raw_post}"