Fix calls to nonexistent functions

Refactorings in the erlang-oauth dependency broke
`couchbeam_util:oauth_header/3`, as the called functions aren't
available anymore.

For reference, here the "breaking change" in erlang-oauth:

https://github.com/tim/erlang-oauth/commit/fc5f528cea05b6b0b5e3ace0300268a330b01ac6