Only check the value for a relationship when the value is a string.

`$faker->datetime` returns a DateTime object, this causes an error to be thrown when it is passed to `preg_match()` in the `hasRelationshipAttribute()` method.
Use `Model:getKey()` for getting the ID for relationships.
Merge branch 'use-getKey-for-getting-id-value'
Merge branch 'check-type-when-checking-for-relationship'