Fix convert_datetime_to_utc() test.

The test had two flaws:
 1. The actual date was different (4/1 vs 1/1)
 2. The timezone usage was not correct (using the timezone as an
    argument of the datetime constructors won't work with timezones
    with daylight saving time according to documentation).
Fix usage of timezones in the documentation.

According to the pytz documentation, using the timezone as an argument
of the datetime constructors won't work with timezones with daylight
saving time, so let's be on the safe side and promote usage of the
localize() method of timezones.