BUG: issue in get_dtype_counts for SparseDataFrame (introduced by dtypes)
- added tests for GH #2873  (commented out)

Noticed intermittent failures in vbench `unstack_sparse_keyspace`.

https://travis-ci.org/pydata/pandas/builds/4808659
https://travis-ci.org/pydata/pandas/builds/4822850
 I have seen this error too
i doesn't have to do with this pr
(as it has happened 'sometimes' on several of PRs )
must be something specifc in the vbench maybe?
 ok then. I'm guessing some randomly generated data is sometimes causing 
a non-unique index. will look when I get a chance.
