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.

https://github.com/laravel/framework/issues/3693
Merge pull request #4288 from skovacs1/4.1

[Bug] Fix many to many polymorphic pivot updates