API: flex comparisons DataFrame vs Series inconsistency
In tests.frame.test_arithmetic we test the DataFrame part of the following, but we do not test the Series behavior (at least not in that file)

```
arr = np.array([np.nan, 1, 6, np.nan])
arr2 = np.array([2j, np.nan, 7, None])
ser = pd.Series(arr)
ser2 = pd.Series(arr2)
df = pd.DataFrame(ser)
df2 = pd.DataFrame(ser2)

ser < ser2  # raises TypeError, makes sense
df < df2  # raises TypeError, makes sense

ser.lt(ser2)  # raises TypeError, makes sense

>>> df.lt(df2)
       0
0  False
1  False
2   True
3  False
```

The `df.lt(df2)` version (the "flex" op) masks positions where either `df` or `df2` is null.  That is fine, but why doesn't the `Series` version do the same thing?