I think this causes a memory fault when running ```iris.tests.unit.coords.test_Coord.Test_collapsed__serialisation.test_serialize```.
```
>>> import mock
>>> import numpy as np
>>> m = mock.MagicMock()
>>> print np.issubdtype(m, np.str)
Memory fault(coredump)
```
I'm writing an addition to this PR and will include ```coord.dtype = points.dtype``` to that test to fix it.