BUG: workaround numpy issue in 1.6.2 on certain distros

pandas issue #2490
numpy/numpy#324

manifests only on certain distors, probably libc dependent.
debian wheezy, libc 2.13-37 is affected
ubuntu precise 2.15-0ubuntu10.2 is not

These are not necessarily the earliest/latest package
revisions addected/not affected respectively.

Author: y-p

pandas/stats/moments.py

@@ -302,7 +302,7 @@ def _center_window(rs, window, axis):
         na_indexer = [slice(None)] * rs.ndim
         na_indexer[axis] = slice(-offset, None)
 
-        rs[tuple(rs_indexer)] = rs[tuple(lead_indexer)]
+        rs[tuple(rs_indexer)] = np.copy(rs[tuple(lead_indexer)])
         rs[tuple(na_indexer)] = np.nan
     return rs