Closes#1978.

3 steps here:

• 7b61ae6 removes a hack to handle gitlab's non-standard headers. That was only valid for 13.0 and fixed in 13.1 - super old now and I wouldn't say we're expected to support that.
• 31c9c2f refactors the nested code out into a slightly more generic helper (that's where it should live anyway, as most of the attributes are from the Gitlab instance)
• d71c1db adds it toauth() so the user gets a warning ASAP.

Other than that I haven't changed any of thestr/None handling to keep thenext_url code the same in_query.