Update Ajax.js

Check if contentType **begins with** 'application/json' instead of checking if contentType **is** 'application/json'.

Otherwise it sets the post body to '[Object object]' when contentType is set to 'application/json; charset=utf-8'.
Match application/json even if it is followed by any other options followed by a semi-colon
Merge pull request #249 from andho/patch-1

Bug with contentType: 'application/json; charset=utf-8'

Reviewed-By: Ben Combee (ben.combee@lge.com)