fix missing constant formatter

JsonApiClient#333

Author: senid231

lib/json_api_client.rb

@@ -2,14 +2,14 @@
 require 'faraday_middleware'
 require 'json'
 require 'addressable/uri'
+require 'json_api_client/formatter'
 
 module JsonApiClient
   autoload :Associations, 'json_api_client/associations'
   autoload :Attributes, 'json_api_client/attributes'
   autoload :Connection, 'json_api_client/connection'
   autoload :Errors, 'json_api_client/errors'
   autoload :ErrorCollector, 'json_api_client/error_collector'
-  autoload :Formatter, 'json_api_client/formatter'
   autoload :Helpers, 'json_api_client/helpers'
   autoload :Implementation, 'json_api_client/implementation'
   autoload :IncludedData, 'json_api_client/included_data'