Replace hardcoded 'declaredStrongAttrs' string with variable.
Replace hardcoded 'declaredStrongAttrs' string with variable.

This makes changing the keys to something that isn't easily collided
easy.

kategengler/ember-strong-attrs,yapplabs/ember-strong-attrs,yapplabs/ember-strong-attrs,jhliberty/ember-strong-attrs,jhliberty/ember-strong-attrs,kategengler/ember-strong-attrs