Bug with contentType: 'application/json; charset=utf-8'

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'.

Enyo-DCO-1.1-Signed-off-by: Amjad Mohamed <andhos@gmail.com>