Resume the queue, setting `content` to be used in the next operation. See `wait()` for an example.
function(content) {
      this.waiting = false;
      if (typeof content !== 'undefined') {
        this.previous_content = this.content;
        this.content = content;
      }
      if (this.callbacks.length > 0) {
        this.then(this.callbacks.shift());
      }
    }