core: Tweak generated ISO8601 dates to appease Google’s parsers

Even though the protocol documentation claims to accept RFC3339 dates,
the servers have recently started rejecting dates formatted as
‘2014-08-09T21:10:05Z’ which don’t include microseconds and specify the
timezone non-numerically.

Since the issue hasn’t been resolved at Google’s end, we need to hack
the date formats generated by libgdata to appease the servers,
completely undermining the point of having a defined protocol and RFC to
base the code on. Sigh.

Change gdata_parser_int64_to_iso8601() to produce dates in the format
‘YYYY-MM-DDTHH:MM:SS.000001+00:00’. This was previously used by the JSON
output generator, so perhaps the server breakage was introduced when
Google started rolling out the JSON APIs.

https://bugzilla.gnome.org/show_bug.cgi?id=732809