fix($httpBackend): remove Content-Type header on DELETE requests

[muratcorlu]

Sending empty string with XMLHttpRequest's post method causes browsers adding unneeded Content-Type request header on DELETE requests. Chrome adds Content-Type:application/xml, Firefox and IE adds Content-Type text/plain. Sending null insted of empty string fixes the problem.

More detail: #2149 (comment)

[mary-poppins]

Thanks for the PR!

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[muratcorlu]

I signed CLA about 30 minutes ago. My real name is Murat Çorlu.

[jroper]

This is duplicated by my PR: #4021

My PR includes a unit test.

[btford]

Closing in favor of #4021

[kevin-king]

I ran into a similar problem and had to set the message body to null, not the Content-Type header to null.

$http({
method: 'DELETE',
data: null,
url: ...
})