While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
* Fix for the BIFF-8 Xls colour mappings in the Reader
* Unit test for reading colours, writing hen rereading and ensuring that the RGB values have not changed