client.isConnected does not return valid status

Example code:

```
if (client && client.isConnected) {
client.disconnect()
}
```

Sometimes throws an exception:

`AMQJS0011E Invalid state not connecting or connected.`

* Chrome 70.0.3538
* mac OS 10.14.1

Start by reproducing the example using client.isConnected and client.disconnect in the reported Chrome and macOS environment. Trace the state checks around these entry points and define done as isConnected reliably reporting a usable connection state without the AMQJS0011E exception during disconnect.

Written by the indexing model from the issue text.