[Bug] Fix many to many polymorphic pivot updates

`$morphType ` set by `setMorphType` is being included in queries from x->pivot->save() and the like. As the object is being added as a public data member on an object with an empty original, it is treated as an attribute and added as a column to the queries. As this column doesn't exist, the query will be erroneous. Adding it as a protected data member hides it from being added as an attribute.

https://github.com/laravel/framework/issues/3693

This still does not resolve the defect that all columns are being submitted because the pivot object has an empty original, but at least that doesn't generate errors.