[tests] directly monkeypatch requests Session

need for later changes
add a tls_prefix parameter to utils.request

This assumes that a backend only ever connects to a single domain, because we
set the fingerprint on the whole session
enable tls_fingerprint for the dav backend
add tls_fingerprint support to the http backend
add myself to AUTHORS.rst
s/CONTRIBUTORS.rst/AUTHORS.rst/
add tls_fingerprint to storage.dav.DavStorage
document tls_fingerprint
Merge pull request #102 from t-8ch/tls_fingerprints

Tls fingerprints

Fix #100