scipy.io.loadmat bug fix

This enables loadmat to take immutable sequences and fixes the bug
whereby loadmat empties a list passed to the variable_names kwarg.

Example:  Prior to this patch, the following code would empty  list ```v```, which is annoying when trying to load identical variables from several files in a loop.

```python
from scipyi.io import loadmat

v = ['x', 'y', 'z']
d = loadmat('/path/to/file.mat', variable_names=v)
```