Allow mixed value for addSource

[vincentvis]

Before the stricter typing we used this like this:

$errorObject->addSource('entity', [
     'type' => $this->type,
     'id'   => $this->id,
 ]);

And relied in the processing of validation errors on being able to get error.source.entity.id
This change forces us to use a string as the entity identifier, which would either make us lose the type or id context, or require extra work on the receiving end to split the string value of $type . ' ' . $id given as a string.

[vincentvis]

Closing my own PR after reading the JSON:API spec