Fix ember inflector default rules

In this large-ish commit (https://github.com/emberjs/data/commit/b1d7448be0c2aff866de6449749441fb7e1d4f98), we lost Ember.Inflector.defaultRules. They were set to `undefined` because the wrong name was imported.

Two jsbins showing it working (1.0.0-beta.6) and `undefined` in (1.0.0-beta.7):
  * 1.0.0-beta.6: http://emberjs.jsbin.com/pezix/1/edit?html,js,output
  * 1.0.0-beta.7: http://emberjs.jsbin.com/pezix/3/edit?html,js,output

I added a few tests that ensure the functionality fairly closely matches that described in the docs here:
https://github.com/emberjs/data/blob/v1.0.0-beta.7/packages/ember-inflector/lib/system/inflector.js#L20

Also a small typo fix in the tests.