webdav: correct error message for incorrect Credential header value

The third-party copy implementation accepts an optional Credential
HTTP header that describes from which source the credential (if any)
should be collected.  If an incorrect value is provided then the
error response lists the valid values.

Unfortunately the incorrect values are in upper-case in contrast to
the acceptable values, which are lower-case.

This patch fixes the error message.

Target: master
Patch: https://rb.dcache.org/r/7394/
Acked-by: Gerd Behrmann
Request: 2.10
Require-notes: yes
Require-book: no