TST: check dtypes unchanged when using DataFrame.assign
TST: check dtypes unchanged when using DataFrame.assign

* Add test to check dtypes unchanged when using assign (https://github.com/dask/dask/issues/3907)
blaze/dask,dask/dask,blaze/dask,jakirkham/dask,jakirkham/dask,ContinuumIO/dask,dask/dask,ContinuumIO/dask,mrocklin/dask,mrocklin/dask