is_utc() returns False when parsing from a UTC date

This date is UTC: '2018-06-21T09:30:00+00:00', however, the is_utc() method returns False.

>>> pendulum.parse('2018-06-21T09:30:00+00:00').timezone
Timezone('+00:00')

>>> pendulum.parse('2018-06-21T09:30:00+00:00').is_utc()
False

The current implementation is:

    def is_utc(self):
        return self.timezone_name == 'UTC'

Maybe it can be something like:

    def is_utc(self):
        return self.timezone.offset == 0

[dazza-codes]

Yes, reproducing the behavior in 1.5.1 vs 2.x is important for me (and possibly others landing here).

[redlickigrzegorz]

I created a pull request #295 for resolving this issue. I hope it resolves the problem. I would like to ask for a review also.