BUG: DataFrame._reduce was converting integers to strings in mixed-type case.
closes #6806

pls add a release note and good 2 go
 @jreback g2g
 k....in general also nice to rebase when adding something (don't worry in this case though). as it makes merging very easy.
 I thought I did rebase and squash. Well, it should be rebased from an hour or two ago.
 I had just pushed something.....:)
 thanks!
 lol. I had to wait for travis to ping green! /me searches for some github lock command. 
 what I usually do is see that the auto-merge is indicated, if not, pull, fix, and repush
