AMS modelNameFromPayloadKey and serializePolymorphicType cleanup

[artych]

This is planned cleanup due to Ember.String functions now works properly with namespaced modelNames, see emberjs/ember.js#10980

[wecc]

LGTM. Would you be able to provide this to https://github.com/ember-data/active-model-adapter too?

[artych]

@wecc , sure

[wecc]

Thanks!

[artych]

@wecc , cc @fivetanley
I've just tried to post PR on ember-data/active-model-adapter repo, but this commit fails test, because it needs following dependencies:

• "ember-inflector": "^1.6.2", (includes commit that fixes singuralize/pluralize for namespace modelNames)
• "ember":"^1.13.0", (includes commit that fixes capitalize/camelize/dasherize/undercore for namespaced modelNames)