BUG: to_dense is broken

[gfyoung]

As of 1d6dbb, the behaviour, documentation, and signatures for to_dense are widely inconsistent:

SparseArray: documentation is incorrect (we are not converting a SparseSeries), and the fill parameter is not even respected if you trace the code. Not sure why it was ever there in the first place.

SparseSeries: What is the purpose of sparse_only? Not only is it not documented, but it also is inconsistent with SparseArray behaviour.

SparseDataFrame: Perhaps the most intuitive of them all. Documentation is correct, and it has no unnecessary parameters like the other two do.

I propose that we change the to_dense behaviour to match that of SparseDataFrame. Take no arguments and just convert to a dense object with no questions asked.

[jreback]

yes I agree these should all be consistent as .to_dense()

[sinhrks]

It's nice to once organize related definitions, including .values, .get_values(), internal_values() and external_values(). maybe internals.rst?