Let blueimp return a text/plain response to old browsers

Currently, the blueimp uploader does not work on old browsers like Internet Explorer 9. A download dialog will popup for the JSON response.

The reason is mentioned in the plugin documentation:

> Iframe based uploads require a Content-type of text/plain or text/html for the JSON response - they will show an undesired download dialog if the iframe response is set to application/json.

This pull request incorporates the suggested ["Content-Negotiotation" workaround](https://github.com/blueimp/jQuery-File-Upload/wiki/Setup#content-type-negotiation) from the plugin documentation.

If the client does not send the HTTP_ACCEPT header including application/json, then a text/plain response will be sent back. The tests have been modified accordingly. A new test has been added to check the content-type in case the HTTP_ACCEPT header does not include application/json.

I hope this pull request will be merged upstream. It would make deploying my own application with the fix easier. :) By the way, thanks for your work on this bundle! It is very solid and has saved me a lot of time.