BUG: to_dense now preserves dtype in SparseArray
Also fixes values and get_values.

fixes #10648

@jreback sorry for being slow in getting back to this, I'll make the changes requested before the week is out.
 can you update
 @ebolyen can you update according to comments. need a whatsnew note as well.
 can you update according to comments
 closing, but pls reopen if you'd like to work on this again.
