* sandbox.pyem: fix a bug which prevents full covariance from being used +...

* sandbox.pyem: fix a bug which prevents full covariance from being used + tests to avoid this issue from being unnoticed

From: cdavid <cdavid@cb17146a-f446-4be1-a4f7-bd7c5bb65646>

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@91 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8