Make Ember.Object.createWithMixins behave like extend().create().

`Ember.Object.createWithMixins` is supposed to emulate
`Ember.Object.extend(SomeMixin).create({foo: bar})`, but since it
interpreted all arguments as a Mixin it causes many small
inconsistencies (including the error reported in #3713).

Once the issue reported in #3713 was handled, I began reviewing the
remaining failing tests. The majority of them were related to
`ReducedComputed` which was using `createWithMixins` to handle computed
properties, but that is explicitly forbidden with `Ember.Object.create`
(see
[here](https://github.com/emberjs/ember.js/blob/8a10e4f284508239772067a37d726991bbc2e36c/packages/ember-runtime/lib/system/core_object.js#L98)).

There are many other failures in the
`packages/ember-runtime/tests/legacy_1x/` directory that I am still
working through.