Spel expressions should run through conversions.

[schauder]

During the review of #2079 it became clear that Spel expressions once evaluated should also pass through the standard conversion.

One typical use case is that a query takes an entity as argument and Spel is used to extract the various members, which might have a custom WriteConversion associated. Such conversion should then get applied.

[mipo256]

As agreed with @schauder, I'll handle this one as well.