Fix a Python 3 incompatibility in the new GitLab tests.
Fix a Python 3 incompatibility in the new GitLab tests.

This is a trivial change that switches a bytestring header to a native
string header for `Link`.

chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard