TST: SparseSeries.density shouldn't die
BUG: SparseSeries.density shouldn't die GH2803
Merge branch 'GH2803'

* GH2803:
  BUG: SparseSeries.density shouldn't die GH2803
  TST: SparseSeries.density shouldn't die
DOC: RELEASE.rst is a full changelog, v0.x.y.txt is what's new/changed