Make compatible with XHR 2

[codler]

Don't force sending the header content-type when doing a ajax call. Sending content-type in header makes it not possible to upload files in XHR 2 because it overwrites this 'multipart/form-data; boundary=----XXX' what would have been automatically sent by the browser.

[Rick Waldron]

Thanks for taking the time to contribute to the jQuery project! Please provide a reduced jsFiddle test case to help us assess your ticket!

Additionally, test against the latest jQuery release and the jQuery 0 GIT version to ensure the issue still exists. Be Excellent to eachother!

Keep in mind, the jQuery 0 Git version includes the latest Ajax re-write

[codler]

It is still the same in the latest jQuery 0 GIT version.

Here is the test ​http://jsfiddle.net/sf3Pb/

[snover]

Force strict false value in contentType to never set Content-Type header.

[jaubourg]

Fixes #7881. Setting contentType to false will prevent the Content-Type header from being sent. Unit test added.

Changeset: 69497c3fd7ff560be0e47b4c65076915cca756bc