Add optional options field to enum_field.rb. See ruby-protobuf#415.

Author: aslakhellesoy

lib/protobuf/field/enum_field.rb

@@ -37,7 +37,7 @@ def coerce!(value)
         type_class.fetch(value) || fail(TypeError, "Invalid Enum value: #{value.inspect} for #{name}")
       end
 
-      def json_encode(value)
+      def json_encode(value, options={})
         enum = type_class.enums.find { |e| e.to_i == value }
         enum.to_s(:name)
       end