Allow multiple headers with the same name to be retrieved from a HTTP Promise

[ChrisPearce]

Responses may sometimes include multiple headers with the same header name. For example, a HTTP response may include multiple "www-authenticate" headers.

Under this scenario, the current way of retrieving the header values (e.g. calling headers('www-authenticate') from within the function ".error(function(data, status, headers, config) {" ) only retrieves the first header.

[Narretz]

Are you sure you are only getting the first header? The way the parser is built, you should be getting a comma-separated string as value.

edit: this is difficult to test, because httpBackend.expect doesn't allow duplicate header keys

[pkozlowski-opensource]

@Narretz is right - the current code in master seems to be handling this situation correctly.
I've opened PR #10176 to add the explicit test.