core: Add helper function to work around Google RFC 3339 incompatibility

In Google Tasks API it isn't supported to provide date/time in
RFC 3339/ISO 8601 format without providing time in miliseconds. As
gdata_parser_int64_to_iso8601() returns a properly formatted string, helper
function replaces last character 'Z' with '.000001+00:00' to fit Google’s
supported format.

https://bugzilla.gnome.org/show_bug.cgi?id=657539