Fix for GitHub issue 632: Empty migrations aren't empty

We now have a ModelBuilder and a BasicModelBuilder. The ModelBuilder has conventions which update the model but the model code generator uses BasicModelBuilder and does not generate anything for ValueGenerationOnSave and ValueGenerationOnAdd. When the snapshot is created these will have default values instead of the correct values, which results in differences between the snapshot model and the current model. The fix generates code for the property value generation settings.