added spec for return from each and each_stored_with_indices (see #87)

Looks like both list and Yale were ok with respect to always returning a value in `each_stored_with_indices`; they return the matrix over which the iteration is happening if a block is provided.  I chose `nil` for the dense return value originally, but perhaps it is worth switching that over to returning the matrix for consistency?

The added specs reflect the current state of the return values for each storage type.