Fix Skipping of Default Values for Extra Props

Values from "extra props", i.e. those where `dontSetInEditor` is set to `YES`, were always written to the file, even if the value was equal to the default serialization.

To fix this, I replaced the different comparisons of `serializedValue` and `defaultSerialization` with a single call to `isEqual` and moved it to a place where it is also called for extra props.