google: 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).