[Time] Solve instanceof problem

Some tests are checking the type of methods return.

For example:
assert_true(returnedValue instanceof tizen.TZDate, "TZDate check");

The way tizen.TZDate was implemented, some methods were returning
instanceof 'undefined'.

Now with this other way to implement tizen.TZDate, the methods are
returning the correct 'instanceof'.
[Time] Fix timezone -> timezone_ mistype
[Time] Fix unecessary check
[Time] Properly format toDateString, toTimeString and toDateTimeString