BUG: Series.map fails when keys are tuples of different lengths (#7333)
closes #7333 seems to be due to the new behaviour of `Series`, which will automatically create a MultiIndex out of the dict's keys when creating a series to map the values. The MultiIndex doesn't match the values for the shorter tuples, so they fail to map.

The fix is fairly simple, just pass the dict keys explicitly as the index.

I've added a test case for this specific issue, but could add some more if others can see related issues that would arise from this. I'll try to finalize the PR (squashing into a single commit, rebasing if necessary) tomorrow if it looks good.

this seems like a bug in the `Series` constructor, i.e., passing tupled-keyed dicts should do the right thing
 probably related to the change to use multiindex for tuple keys by default
 hm nevermind ... i guess the constructor behavior makes sense from a mi point of view
 pls add a release note to `doc/source/v0.14.1.txt` as well
 Alright, I've added to the release notes and it seems like everything's good, so I've squashed into a single commit. Should be okay to merge if people are OK with it.
 @cpcloud looks ok to you? (i'll clear conflict on merging)
 Yep. 
 @onesandzeroes thanks!
