Add auth_type param for ClientCredentials

Allows user to pass in a different authorization header type, defaults
to the original Basic type.
Revert "Merge remote-tracking branch 'upstream/master'"

This reverts commit fd100ce14ac9ad9bac88b6631a070ad6a14091d8, reversing
changes made to 8a45677df533fceaa52ea77d528fd65e25c1e705.