BUG: df.join(df2, how='right') TypeError
Issue #11519

Somehow right joins had been forgotten in a previous bugfix. There were tests already written that should have seen the problem, but they had been commented out because the expected results were wrong, because of a subtlety in the way non-unique index are handled.

Thanks for the labels "difficulty novice" and "effort low": I'm using pandas every day, and I'm glad I could contribute that easily.

Ok, the commented test is redundant with what is just before, so I'm removing it.
 pls add a whatsnew note (bug fixes) & squash
 Done
 can you add all of the examples from the issue as well. (in `tests/tools/test_merge.py`) would be a good location
 I've put them in what I thought was the most appropriate test method, I hope that's all right.
 merged via https://github.com/pydata/pandas/commit/d78266eb396143438d187eb8d5a8cb750c2ba737

thanks!
 @nbonnotte @jreback 

Thanks for closing this out! :+1: 
