SES-2043 : Check for JSONP before Accept to work with Chrome/Angular

Chrome, with Angular at least, seems to send Accept */* for JSONP
requests instead of the typical "application/javascript" and hence the
callback parameter needs to be checked before the Accept header to be
successful.
Merged in SES-2043 (pull request #269)

SES-2043 : Check for JSONP before Accept to work with Chrome/Angular