support for protocol relative url

[nurey]

Seems if I set url to a protocol relative url like "//otherdomain.name/upload_handler" from a site on domain.name, the xhr request is made to domain.name and not otherdomain.name

[nurey]

here's an example to demonstrate the issue:
http://foo.nurey.com/plupload/examples/custom.html

The url is set to //bar.nurey.com/plupload/examples/upload.php but the request is still made to foo.nurey.com

[wdmtech]

This is happening for me in the latest version, plupload is rewriting the url: option to the request's origin or something it seems - when it should be making a cross-origin request to a third-party API instead.

Possibly related:

#919
#910