This reverts commit f280cc1c.

Previously, rack-oauth2 only accepted Symbols for client_auth_method.
Commit f280cc1c worked around this by converting the String type to
a Symbol and forwarding it.

As of 1.14.0, rack-oauth2 accepts both data types. With the version bump
to 1.16.0, the workaround that was in place can now be removed.
Signed-off-by: Vincent Fazio <vfazio@xes-inc.com>