inconsistent dtype conversion during join
Just stumbled about a very nasty subtle dtype inconsistency after join operation that crippled by out-of-core-join-many-result-files pipeline for unfitting index sizes for the HDF append operation.
See the example code here:
https://nbviewer.jupyter.org/gist/michaelaye/1c34cda54872046b5989
or here as screenshot:
<img width="849" alt="screenshot 2015-08-20 15 23 01" src="https://cloud.githubusercontent.com/assets/69774/9396054/5df0dbc6-474f-11e5-9282-eff823d51b18.png">

What happens that the dtypes of the to-be-joined dataframe depend on if the joining dataframe has either 1 or 2 columns.

Any hint how to avoid that would be appreciated.

Version: '0.16.2+148.gc740bb0'
