ARROW-433: Correctly handle Arrow to Python date conversion for timezones west of London

Verified with `TZ='America/New_York' py.test pyarrow`

Author: Uwe L. Korn <uwelk@xhochy.com>

Closes #245 from xhochy/ARROW-433 and squashes the following commits:

06745d8 [Uwe L. Korn] Use more pythonic approach
a55be24 [Uwe L. Korn] ARROW-433: Correctly handle Arrow to Python date conversion for timezones west of London

Author: xhochy

python/pyarrow/scalar.pyx

@@ -127,7 +127,7 @@ cdef class DateValue(ArrayValue):
 
     def as_py(self):
         cdef CDateArray* ap = <CDateArray*> self.sp_array.get()
-        return datetime.date.fromtimestamp(ap.Value(self.index) / 1000)
+        return datetime.datetime.utcfromtimestamp(ap.Value(self.index) / 1000).date()
 
 
 cdef class TimestampValue(ArrayValue):