[5.4] Use setRawAttributes in Pivot constructor to avoid casting twice

[jrhenderson1988]

At present, the Pivot constructor uses forceFill to set the attributes of the pivot model, which uses setAttribute to set each property and ends up casting the data provided as a string given by the database to the relevant type. Most notably, for attributes that are set to be cast as a JSON castable type (array, object etc.) this results in a JSON encoded string from the database being JSON encoded again when it's being set in the Pivot model. Therefore, accessing such an attribute results in a JSON encoded string being returned rather than an array or stdClass object as expected.

This pull request changes forceFill to setRawAttributes so that casting is not carried out when setting the Pivot attributes.

#18135

Note: This PR is causing 2 tests to fail. One which ensures mutators are called from the constructor of Pivot and another which ensures that a dummy date passed in has been mutated. My suggestion intentionally skips the mutators by using setRawAttributes, similar to when a Model is created, therefore I'm not sure what to do regarding this.

[taylorotwell]

I believe this was already fixed but not tag. Regardless, this would be the wrong change to fix it as it would break other scenarios.