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

Closes https://github.com/pydata/pandas/issues/7512.
Replacing:
    window = len(arg1) + len(arg2)
with:
    arg1, arg2 = arg1.align(arg2)
    window = len(arg1)
works for series, but not for data frames: it not only merges the index, but also the columns, thus producing the wrong result for pairwise=True calls -- i.e. if arg1.columns=['A','B'] and arg2.columns=['X','Y'], then while result should have major/minor axes ['A','B'] and ['X','Y'], instead it ends up with both being ['A','B','X','Y'].

At any rate, since the two objects end up being aligned anyway in _flex_binary_moment(), aligning them in expanding_cov/corr is just redundant.

I updated the code to include a coupe additional tests, but left window = len(arg1) + len(arg2) unchanged.