BUG: test case showing why assigning to dtype is unsafe

@jreback this is a test case that shows why assigning to `dtype` is unsafe, namely that the dtype item sizes have to be compatible obviously =P Could you take a look at this and fix? This came up when I booted the windows box back up because Windows uses int32 by default.

There is a secondary bug here that the test case in question probably _should not_ cause the dtype of the resulting Series to change from int64 to float64, agreed?