Reflect X-XHR-Referer in request.referer - closes #169

This is an implementation of the idea proposed in #169.  It ensures that `request.referer` returns the expected url.  

The existing `referer` method is defined on Rack::Request:

```ruby
module Rack
  class Request
    def referer
      @env['HTTP_REFERER']
    end
    alias referrer referer
  end
end
```

This patch defines a `referer` method on ActionDispatch::Request that will return the value of the `X-XHR-Referer` request header if it exists and falls back to the original method if it doesn't:

```ruby
module ActionDispatch
  class Request < Rack::Request
    def referer
      self.headers['X-XHR-Referer'] || super
    end
    alias referrer referer
  end
end
```