Better support of enum mappings

Fixed case:
When we map non-nullable enum field from entity to nullable target field (in my case it was due left join), exception is thrown.

Fix maybe not very pretty, but I don't see better solution.
If we had information about source and target types alltogether - it will be more error-proof. In that case we could convert value to required type in mapper. Now I changed type of data, received from data reader in mapper and fixed two cases, detected by tests on other side, when we assign value to target.