as_datetime() doesn’t really respect tz for character objects

[huftis]

The as_datetime() function doesn’t work as expected for character objects.

Expected behaviour: The datetime string should be interpreted as a datetime in the chosen time zone tz (UTC by default).

Actual behaviour: The datetime string is interpreted as a datetime in the current time zone (including any DST-specific stuff) and then converted to the tz timezone.

Example (as run in the CET time zone):

> as_datetime("2015-01-04 03:10:00")
[1] "2015-01-04 02:10:00 UTC"
> as_datetime("2015-01-04 03:10:00", tz="UTC")
[1] "2015-01-04 02:10:00 UTC"

Note that the time part is an hour off. Expected result:

[1] "2015-01-04 03:10:00 UTC"

[vspinu]

Duplicated by #536 and fixed in cctz branch.