Remove harmonisation of data types

This fails when a belongs to model is set to null, causing no change
to be recorded and thus the database doesn't get updated.

Removing the harmonisation requires the developer to use the correct
data types.
Fix related handling when setting a belongs to relationship to NULL

Setting the value of the related array to NULL broke other parts of
ORM which assumed that the related array only contained ORM
instances.
Merge pull request #1 from kiddivouchers/related-model-fixes

Related model fixes