Series[datetime64tz] - DatetimeIndex[tz] raises ValueError
Opening an issue mostly so the upcoming PR can reference it.  This is in fact already fixed in master (at least as of 93033151a8), so the PR is just a test.

Setup:
```
dti = pd.date_range('1999-09-30', periods=10, tz='US/Pacific')
ser = pd.Series(dti)
```

0.22.0:
```
>>> ser - dti
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 720, in wrapper
    converted = _Op.get_op(left, right, name, na_op)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 349, in get_op
    return _TimeOp(left, right, name, na_op)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 385, in __init__
    self._validate(lvalues, rvalues, name)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 444, in _validate
    raise ValueError("Incompatible tz's on datetime subtraction "
ValueError: Incompatible tz's on datetime subtraction ops

>>> dti - ser
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 720, in wrapper
    converted = _Op.get_op(left, right, name, na_op)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 349, in get_op
    return _TimeOp(left, right, name, na_op)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 385, in __init__
    self._validate(lvalues, rvalues, name)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/ops.py", line 444, in _validate
    raise ValueError("Incompatible tz's on datetime subtraction "
ValueError: Incompatible tz's on datetime subtraction ops
```