[10.x] Set morph type for MorphToMany pivot model

[gazben]

Discussion: #43760

If you have a MorphPivot entity with a custom class, when attaching a record, you get the pivot with all the fields loaded on the model, but if you detach it, you only get the IDs of both sides, so in case of morphPivots, you do not get the type/class of the entity so there's information missing and you cannot obtain the morph relation from the pivot because it throws an error.

With this change the morph type will be automatically added to the pivot attributes, so you can query the relation model from a listener for example.

[taylorotwell]

Hey there - I think we would need tests for this.

[gazben]

@taylorotwell I've added the missing test to check for this use-case. If the newly added array_merge is commented out, the test fails because only the equipmentable_id attribute is present in the deleted() callback.

With the now modified attributes, the query will run.