Do not crash without www-authentication header

[NotSpecial]

Currently, there is an issue with abort(401). Per default,the keyword argument www_authenticate is None for the UnauthorizedException, i.e. abort(401) (as described in the Werkzeug docs), and the resulting exception attribute is e.www-authenticate = (None,).

In endpoints.py#L173, this (None,) is added to the header stack, which crashes response processing in render.py#L164, as (None,) cannot be unpacked to header, value.

In our API, an automated basic auth response does not make sense, yet calling abort(401) (without the www_authenticate) is currently impossible with Eve.

In this PR, I have modified the error endpoint to only add the www_authenticate header if it is a proper header, value pair and not the (None,) default.

I'd love to add a test as well, but I don't quite know where in the test suite this would fit in best. Any pointers are appreciated, and I'll get to it.

[nicolaiarocci]

@NotSpecial are you on Werkzeug 0.14.1 or 0.15.1?

[NotSpecial]

0.15.2 actually, but I don't think they changed abort 401 again after 0.15.1

Edit: Just tried it with 0.15.1, same issue.

[NotSpecial]

Update: added link to the Werkzeug docs, where the default argument www_authenticate=None is described)

[nicolaiarocci]

Thanks.

[NotSpecial]

Awesome, thanks for merging and the fast response!