Explicitly bind `url-privacy-level' to avoid accidentally omitting User-Agent header.

GitHub API always requires a User-Agent header to be sent.
If the Emacs user has decided to configure `url-privacy-level' such
that it will omit User-Agent, gh.el will fail for everything it does.
Merge pull request #31 from mlang/user-agent-required

Explicitly bind `url-privacy-level' to avoid accidentally omitting User-Agent header.