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
True, I didn't write ones specifically for polymorphic relationships as the code path was the same.