Add n kwarg and size tests to ffts.
Add n kwarg and size tests to ffts.

clarkfitzg/dask,jcrist/dask,gameduell/dask,pombredanne/dask,blaze/dask,jakirkham/dask,ssanderson/dask,wiso/dask,mraspaud/dask,blaze/dask,chrisbarber/dask,vikhyat/dask,jakirkham/dask,ContinuumIO/dask,dask/dask,wiso/dask,dask/dask,cowlicks/dask,PhE/dask,pombredanne/dask,jcrist/dask,ContinuumIO/dask,ssanderson/dask,mrocklin/dask,PhE/dask,mrocklin/dask,cpcloud/dask,clarkfitzg/dask,mikegraham/dask,mraspaud/dask,vikhyat/dask