AJAX calls should set the "Accept" header if appropriate

[MikeBaranczak]

Some web service libraries (like Jersey) allow you to send back data in different formats (like XML, JSON or www-form-encoded) from the same URL. The client can select the format by setting the "Accept" request header. This unfortunately can't be done through JQuery.

I propose the following enhancements:

1. Allow the "Accept" header to be set when calling $.ajax();

2. Make $.getJSON() (and possibly other high-level AJAX calls) set this header to the appropriate value by default.

(See the comment by Wolfram Arnold on this page:  http://api.jquery.com/jQuery.getJSON/ )

[MikeBaranczak]

Sorry, please disregard this ticket. It turns out that JQuery already does set the "Accept" header.

[MikeBaranczak]

OK, it looks like this is actually a bug that only appears in some situations. It happens when the URL contains a parameter: 'callback=?' - I haven't seen it happen otherwise. Someone who knows more about JSONP than me should probably review this.

[MikeBaranczak]

This ticket is a duplicate of #6551.

[snover]

Resetting milestone to future.

[snover]

Setting dataType sets the Accept header.