BUG: datetime/timedelta Series quantile() call
Changes to be committed:
    modified:   pandas/core/series.py
    modified:   pandas/tests/test_series.py

Fixes global reference to iNaT (should be tslib.iNaT) in series._maybe_box.
Adds test `test_datetime_timedelta_quantiles` to check for proper return value
in test_series.py.

Issue #9675
