explicitly check the headers to find out if we already set it

Under certain conditions the old check would wrongly report "already
set".
updated test for Auth-Bearer header plugin to actually test the headers
added test for Auth-Bearer header plugin

For the case that the same plugin instance is used over multiple
requests.

This used to break at the second request, see
https://github.com/easybiblabs/oauth2-client-php/pull/10
deleted unused statement
Merge pull request #10 from rwos/bugfix/bearer-auth-header

explicitly check the headers to find out if we already set it