This fixes error "unauthorized_client: invalid client secret" when
client includes secret in Authorization header rather than request body.
OAuth spec permits both.
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>
Fixes#21282
As suggested by the [OAuth RFC](https://www.rfc-editor.org/rfc/rfc6749)
(quoted below), it's helpful to give more detail in the description
> error_description
OPTIONAL. Human-readable ASCII
[[USASCII](https://www.rfc-editor.org/rfc/rfc6749#ref-USASCII)] text
providing **additional information, used to assist the client developer
in understanding the error that occurred.**
Values for the "error_description" parameter MUST NOT include characters
outside the set %x20-21 / %x23-5B / %x5D-7E.
