BUG: {expanding,rolling}_{cov,corr} don't handle arguments with different index sets properly

Closes https://github.com/pydata/pandas/issues/7512.
The expanding_xxx() functions need window = len(arg1) + len(arg2) rather than = max(len(arg1), len(arg2)) because once arg1 and arg2 are merged the result could be of length up to len(arg1) + len(arg2) if there is no overlap between the index (or major_axis) of arg1 and arg2. If the index of one is a subset of the index of the other, then max() will be fine; but in general when neither is a subset of the other max() will be too small.