Properly remove authentication to download assets

There is a great deal of complexity in requests surrounding how headers
are managed and merged. Since the authentication is applied after
headers are merged, using basic authentication can still be applied.
Using this context manager ensures it will not be set again.

Fix #288

(cherry picked from commit 7b3f589d36b151a05536d2ffb02d2ac192629707)

Conflicts:
	tests/unit/test_repos_release.py