Check meta in `__init__` of _Frame
Check meta in `__init__` of _Frame

Add check on initialization of `_Frame` objects to ensure the metadata
is of the proper type.

cowlicks/dask,dask/dask,blaze/dask,mrocklin/dask,jakirkham/dask,cpcloud/dask,mraspaud/dask,jcrist/dask,ContinuumIO/dask,gameduell/dask,jakirkham/dask,chrisbarber/dask,dask/dask,mraspaud/dask,ContinuumIO/dask,jcrist/dask,mrocklin/dask,blaze/dask