Non-compliant mtime format in "getlastmodified" Webdav field

[PVince81]

In Phoenix, the moment library complains that the returned format is not compliant:

The Webdav property getlastmodified must respect rfc1123: https://tools.ietf.org/html/rfc4918#section-15.7

I found something related here: https://stackoverflow.com/q/25658897

Apparently one cannot use "UTC" as a timezone there and should return either "UT" or "GMT".

[PVince81]

Apparently we're using standard formatting: https://github.com/cs3org/reva/blob/9358716769b0a7f5f28e6e7aa44a6e747615ca0b/internal/http/services/owncloud/ocdav/propfind.go#L241

and it seems to not be 100% compliant: golang/go#13781

[PVince81]

Either we pre-process it on the Phoenix side to avoid that warning, or we pre-process the values in ocdav before returning it. I'd be more in favor of the latter.

[PVince81]

I reread the linked ticket and apparently we should use RFC1123Z instead.

Here's a PR that does it cs3org/reva#805 and indeed fixes the format issue.