Parameter for Custom Headers

[Naomarik]

I'm not entirely sure if this is desired following sentiment of the discussion here: #515

I've hacked it in myself

Turbolinks.visit(url, change: ['search_page'], header: { key: 'X-Render-Partial', value: 'search_page'})

In the fetchReplacement method

xhr.setRequestHeader 'X-XHR-Referer', referer
# Under this line
if options.header
  xhr.setRequestHeader options.header.key, options.header.value

And this lets me call this.

if partial = request.headers['X-Render-Partial']
  render partial: partial, layout: false and return
end

This has allowed me to whittle my views render time from ~500ms...

...to ~55ms for the same query by avoiding the layout rendering and choosing the exact partial to render.

Causing my app to be the fastest thing I've ever seen.