Fix coerce_to_pendulum() for python datetime.date

[martinburchell]

coerce_to_pendulum() was returning a naive pendulum DateTime if given a standard python datetime.date object. This is because:

• The datetime.date object has no concept of time zone
• The call to Pendulum DateTime.combine() to add the time component returns a naive Pendulum DateTime
• pendulum.instance() ignores the tz argument if given a Pendulum DateTime

This fix replaces the call to Pendulum DateTime.combine()with the standard python datetime.combine(), which means that the tz argument won't be ignored

[RudolfCardinal]

Interesting -- thanks! Worth bumping the version and updating the changelog at this point?