Update DataAdapterMixin docs (#1366)
Update DataAdapterMixin docs (#1366)

* Update data-adapter-mixin.js

Fix incorrect docs about headersForRequest being used for ember-data versions >= 2.7.
This is on hold until https://github.com/emberjs/rfcs/pull/171 gets resolved

* Update data-adapter-mixin.js

simplabs/ember-simple-auth,simplabs/ember-simple-auth,stevenwu/ember-simple-auth,omniboard/ember-simple-auth,clairton/ember-simple-auth,omniboard/ember-simple-auth,stevenwu/ember-simple-auth,clairton/ember-simple-auth