BUG: Fixes #5376: np.ravel to return same array type

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.
Two tests which were expecting np.ravel to return an array have been changed.
Also, the optional `order` parameter was added to MaskedArray.ravel to make it compatible
(matrix.ravel already had this parameter).