Fix bug #10: OAuth does not work on ovh.com server

Fix bug #10: OAuth does not work on ovh.com server

This ovh.com server always has $_SERVER['HTTP_AUTHORIZATION'] set.
By default it has an empty string as value, and this breaks the PHP
OAuth extension - see https://bugs.php.net/bug.php?id=68168

We work around that by forcing the signature method in this special case,
so that no exception is thrown.

Thanks to Julien Daviaud for giving me access to his server for debugging.