Fix MorphToMany relation not using Eloquent\Model@getMorphClass() from b41bf57f65f32572ba3113910213f4d1077a7578

Signed-off-by: Jason Urasky <jurasky@aiaonline.org>
Merge pull request #6317 from jurasky/bug/eloquent-morphtomany

[4.2] Fix MorphToMany Relation Not Using Eloquent\Model@getMorphClass()