Handle HTTP 302 without Location

[denisdefreyne]

Fixes #1297.

A HTTP 3xx header without Location is now an explicit error:

broken reference to http://example.com/x: redirection without a target location

[iay]

Silently ignoring it is better than crashing, for sure. However, I think if the response is out of spec because the Location is missing (and I should really look at the spec) then I'd want it to be flagged as an error rather than ignored. After all, if it's broken in that way my reference to it isn't likely to be intentional; if it is, I can always exclude it from the checks.

[denisdefreyne]

@iay I just changed the behavior — probably while you were typing — to raise an error in that case!