allow polymorphic relationship types to not exist

Previously, you had to have a model defined
for polymorphic relationships. However, the
polymorphic relationship belongsTo/hasMany
did not actually have to be an instanceof the
base type declared in DS.belongsTo(‘modelType’).

This commit removes that check for polymorphic
types only.

fixes GH-2212
I am not sure this is the best place to do this. Seems like modelFor should always look for the type. Maybe we can use _modelExistsFor from #2346 ?