Use OAuth app auth as basic auth without creating a new client

Taking a stab at what was discussed in https://github.com/octokit/octokit.rb/pull/397#issuecomment-33239274 and https://github.com/octokit/octokit.rb/pull/397#issuecomment-33263810.

Allows `:basic_oauth_app_auth` to be passed in requests which uses the
already configured OAuth application authentication as basic
authentcation for certain endpoints that need it.