Allow belongs to associations to be saved when the associated object is null.
Remove accidentally added capitalize and camelize
Merge branch 'fix-polymorphic-belongs-to-null' of github.com:Tiwalun/data into Tiwalun-fix-polymorphic-belongs-to-null

Conflicts:
	packages/activemodel-adapter/lib/system/active_model_serializer.js
	packages/ember-data/lib/serializers/rest_serializer.js
	packages/ember-data/tests/integration/serializers/rest_serializer_test.js
Merge branch 'Tiwalun-fix-polymorphic-belongs-to-null'