def call(env)
      status, headers, body = @app.call(env)
      headers['X-Node'] = @host if @host
      headers['X-Revision'] = @sha
      [status, headers, body]
    end
def call(env)
      status, headers, body = @app.call(env)
      headers['X-Node'] = @host if @host
      headers['X-Revision'] = @sha
      [status, headers, body]
    end
Initializes the middlware.

 app     - The next Rack app in the pipeline.
 options - Hash of options.
           :host      - String hostname.
           :revision  - String SHA that describes the version of code
                        this process is running.

 Returns nothing.