Backport #21351
This fixes error "unauthorized_client: invalid client secret" when
client includes secret in Authorization header rather than request body.
OAuth spec permits both:
https://www.rfc-editor.org/rfc/rfc6749#section-2.3.1
Clients in possession of a client password MAY use the HTTP Basic
authentication scheme ... Alternatively, the authorization server MAY
support including the client credentials in the request-body
Sanity validation that client id and client secret in request are
consistent with Authorization header.
Improve error descriptions. Error codes remain the same.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
