Unstack with an object variable overwrites BlockManager

[jseabold]

Consider the (long) code snippet here. Unless you remove ccode from the df beforehand, you end up with all object dtype for df after unstack.

http://www.pastie.org/2904328

[wesm]

took care of this in the above commit. was less trivial than I expected

[jseabold]

Awesome. Thanks.