Add withDefault to MorphOne relations.

[sileence]

This PR adds the default option for morph one relations.

I also moved the logic to a trait to avoid code duplication. The only difference between morphOne, hasOne and belongsTo was the code to create the related instance, so I extracted that to an abstract method. So in order to implement the withDefault feature in a relation, you need to include the trait HasDefault and then implement the newRelatedInstanceFor method. I hope this makes sense.

[sileence]

cc @browner12 @decadence

[decadence]

Thanks