hasForwarder(forwarder) {
    return this._forwarders.includes(forwarder);
  }
Checks whether the specified [`Forwarder`](Forwarder) object has already been attached to this
input.

@param {Forwarder} forwarder The [`Forwarder`](Forwarder) to check for (the
[`Forwarder`](Forwarder) object is returned when calling [`addForwarder()`](#addForwarder).
@returns {boolean}