Fix many to many polymorphic pivot updates

MorphType set in setMorphType is being included in queries to update the pivot using `x->pivot->save()` to affect pivot data. As this means queries to affect pivot data will look for a morphType column, the query will be erroneous. Adding it as a protected member prevents this inclusion.

https://github.com/laravel/framework/issues/3693

This still does not resolve that queries will still submit all columns because the original object is empty and so all columns are treated as dirty.