fix(date): invert timezone sign and always display sign

This commit fixes #1261 and #1532. This covers
two separate issues:

- Positive timezones were being formatted without
a leading `+` resulting in a formatting string
like: "HH:MM:ssZ" giving "12:13:141000" instead
of "12:13:14+1000". Fixed by checking if timezone
is > 0 and adding a leading "+".

- Timezone output signs were inverted.
mock.TzDate expects the timezone _offset_ as it's
first argument, _not_ the timezone. This means
that a mock.TzDate with a positive offset should
result in a date string with a negative timezone,
and vice-versa.

Closes #1261, #1532