Close resque#576, Close resque#573, Close resque#571, Close resque#570

This commit uses feature detection with MultiJson so we don't have
to lock to specific version which would impact other projects.

The commit also handles the edge case where OkJson is loaded
and may be referenced in an unknown namspace.

Idea: https://github.com/jonleighton/poltergeist/pull/52/files

lib/resque/multi_json_coder.rb

@@ -3,21 +3,32 @@
 
 # OkJson won't work because it doesn't serialize symbols
 # in the same way yajl and json do.
-if MultiJson.engine.to_s == 'MultiJson::Engines::OkJson'
-  raise "Please install the yajl-ruby or json gem"
+
+if MultiJson.respond_to?(:adapter)
+  raise "Please install the yajl-ruby or json gem" if MultiJson.adapter.to_s == 'MultiJson::Adapters::OkJson'
+elsif MultiJson.respond_to?(:engine)
+  raise "Please install the yajl-ruby or json gem" if MultiJson.engine.to_s == 'MultiJson::Engines::OkJson'
 end
 
 module Resque
   class MultiJsonCoder < Coder
     def encode(object)
-      ::MultiJson.encode(object)
+      if MultiJson.respond_to?(:dump) && MultiJson.respond_to?(:load)
+        MultiJson.load object
+      else
+        MultiJson.encode object
+      end
     end
 
     def decode(object)
       return unless object
 
       begin
-        ::MultiJson.decode(object)
+        if MultiJson.respond_to?(:dump) && MultiJson.respond_to?(:load)
+          MultiJson.dump object
+        else
+          MultiJson.decode object
+        end
       rescue ::MultiJson::DecodeError => e
         raise DecodeException, e.message, e.backtrace
       end