fix: convert dates for propfinds from RFC3339 (ocis) to RFC1123 (ocdav)

[kulmann]

While oCIS uses RFC3339 time formats internally, we use RFC1123 time formats for webdav responses. Since oCIS doesn't know about the format, we now convert the time strings that arrive in reva from RFC3339 to RFC1123.

[butonic]

since the property is exposed via WebDAV one could assume that RFC1123 is necessary. Actually, the getlastmodified property is sepcified as RFC1123, while creationdate is RFC3339.

getlastmodified is derived from the Last-Modified header, which is ancient and thus has to remain RFC1123 for backwards compatability.

This does not apply to properties we control.

web should expect RFC3339 for any date-time properties with the exception of well defined RFC1123 properties like getlastmodified.

Stick to RFC3339.

[kulmann]

Closing as not needed. Even better if we can stick to RFC3339. Thanks @kobergj and @butonic !