`np.ravel` should return the same array type it was given

If given a matrix, then `np.ravel` should return a matrix.  And similarly for a masked array.  Currently, it always returns an ndarray.

In PR #5358, `np.diagonal` was modified to return whatever array type it took in.  Also, `np.cumsum` and `np.clip` return the same array type.  So, `np.ravel`'s behavior is surprising.
