Refactor JSONAPISource#ajax.
Refactor JSONAPISource#ajax.

* Uses `settings` instead of `hash`.
* Sets headers via `settings` instead of `setRequestHeader`.
* Passes `settings` as an arg to `ajaxHeaders` and `ajaxContentType`.
orbitjs/orbit-core,jpvanhal/orbit-core,orbitjs/orbit.js,SmuliS/orbit.js,orbitjs/orbit.js,SmuliS/orbit.js,jpvanhal/orbit-core,jpvanhal/orbit.js,jpvanhal/orbit.js