fix touch() when $mtime is set (Google wants RFC3339) #11267

ownCloud passes us a Unix time integer, but the GDrive API wants
an RFC3339-formatted date. Actually it wants a single particular
RFC3339 format, not just anything that complies will do - it
requires the fractions to be specified, though RFC3339 doesn't.

This resolves issue #11267 (and was also noted by PVince81 in
reviewing PR #6989).