BUG: DatetimeIndex uses the wrong union if the operands overlap

This pretty much boils down to this line: https://github.com/pydata/pandas/blob/master/pandas/tseries/index.py#L987

You can easily reproduce the error using the following code:

    r = pd.date_range("2013-01-01", "2013-02-01")
    r2 = r + pd.DateOffset(minutes=15)
    r | r2 # Results in the index r having "2013-01-31 00:15" appended

This happens because of the wrong direction of the check in the given line, it should say `<=` instead of `>=`, because then `_can_fast_union` reports that the indices can be simply adjoined although they are not disjoint.