I added a test for too few observations.

```python
    def test_spearman_too_few_obs(self):
        """Test that spearman performs correctly with too few observations."""
        self.assertRaises(ValueError, spearman, [], [])
        self.assertRaises(ValueError, spearman, [], [1,2,3,4])
        self.assertRaises(ValueError, spearman, [1], [1])
```