byebye message not sent when calling bus.stop()

HI @achingbrain,
I'm using your library, and integrating it with express, as per your example:
`const upnp = require('@achingbrain/ssdp')(options.ssdpOptions);`
`const ssdp = require('./ssdp/ssdp');`
`upnp.on('error', console.error) // Log errors`
` .advertise(ssdp.services) // Advertise the services configured`
` .then(ssdp.serviceHosting) // Configure Express to serve the details.xml file`
` .then(addRoutes); // then add all the other routes`

where:

- options.ssdpOptions specifies a guid for the udn,
- ssdp.services contains the USN, the location and the details for the service,
- ssdp.serviceHosting contains the routing for express, and
- addRoutes adds all the remaining routes to express.

To get to this point, I needed to modify stop-advert.js slightly. To integrate with express, I need to specify a location, but if the location is specified, no shutdownServers function is created in create-location.js, and an unhandled promise rejection is thrown in stop-advert.js when it tries to call plumbing.shutdownServers. All I'm doing is checking if shutdownServers exists before it's called.

The issue I'm having is when I call upnp.stop(), a transport:outgoing-message is emitted for the byebye, but I never see the corresponding packet in wireshark. The socket.send call never seems to return.

I can submit a PR for the unhandled promise rejection, but I'm stuck on the other issue. Any ideas?

Start by tracing upnp.stop() through stop-advert.js and the socket.send call, then compare the emitted transport:outgoing-message with the packet observed in Wireshark. Also inspect create-location.js and the shutdownServers path for the unhandled rejection. Done means the byebye packet is sent successfully and stopping with a configured location does not reject unexpectedly.

Written by the indexing model from the issue text.