Fixed two probems in access token check

1) The method returned the wrong result in case when the access token itself contained the string "Bearer".

2) When using cURL, the request is sometimes send twice (in my case when the first request returned a 404 error), and the Authorization header of the second request is doubled, so that you get a "Authorization: Bearer XXX, Bearer XXX". This case is checked now. (BTW: Tested with the current PHP version 5.4.15 on Windows.)