BUG: fix NDFrame.as_blocks() for sparse containers

SparseBlocks don't consolidate, so previous implementation silently
dropped all but the last blocks for given dtype.

Also, drop the 'columns' kwarg, there's reindex for that.
Merge pull request #6748 from immerrr/fix-as-blocks-for-sparse

BUG: fix NDFrame.as_blocks() for sparse containers