BUG: {expanding,rolling}_{cov,corr} don't handle arguments with different index sets properly
Closes https://github.com/pydata/pandas/issues/7512.

needs a release note in v0.14.1 (in bug fix section)
 ok, just ensuring that this is a bug fix only (and not actually changing the result; which is technically wrong currently when the indices are not aligned).
 thanks
