Fix EZP-20979: content integration tests

This PR resolves issue https://jira.ez.no/browse/EZP-20979

Integration tests are implemented for creating and updating Content with passing reduced (not redundant) field set to the storage engine.
Tests are not to be run with memory stub implementation and are in separate suite.

Problems fixed during testing:

* fields were not created for main or initial language if removing redundant fields left no fields for that langauge
* Integer field type converter possibly passed `null` for sortKeyInt, which is not nullable
* Integer field type converter did not restore sort key for default value in field definition
* field type was not registered in SPI integration test for BinaryFile