Per RFC 4918 section 10.6, when the Overwrite header is omitted from a
COPY or MOVE request the resource MUST treat the request as if
Overwrite: T had been sent.
The upstream golang.org/x/net/webdav library mishandles this for MOVE
by checking == "T" instead of != "F", so an absent header is treated
as Overwrite: F and the request fails with 412 Precondition Failed.
Normalise the header to T in the rclone WebDAV server before
delegating to the upstream handler when the client did not send one.
This restores RFC-compliant default behaviour and can be removed once
the upstream fix in golang/go#66059 lands and the golang.org/x/net
dependency is bumped.
Fixes#9496
(cherry picked from commit cfb9a10a3d)
39487c5ebf