title: ngx-mqtt service options error for protocol
body: <p>In my application i am using angular 7 and the ngx-mqtt package "ngx-mqtt": "^6.8.3". The application works over https so secure connection is used on MQTT server too.</p>
  
  <p>this is my environment.ts</p>
  
  <pre><code>MQTTCONFIG: {
      broker: 'theBroker',
      hostname: 'theHostname',
      **protocol: 'wss'**,
      port: thePort,
      username: 'theUsername',
      password: 'thePassword',
      path: 'thePath',
      topic_query: 'theTopicQuery',
      topic_update_state: 'theTopicUpdateState',
      clientID: 'smartorder'
    }
  </code></pre>
  
  <p>and this is my app.module.ts (some declarations and imports are omitted):</p>
  
  <pre><code>import { MqttMessage, MqttModule, MqttServiceOptions } from 'ngx-mqtt';
  .
  .
  @NgModule({
    declarations: [
      AppComponent
    ],
    imports: [
      BrowserModule,
      FormsModule,
      ReactiveFormsModule,
      BrowserAnimationsModule,
      AppRoutingModule,
      MaterialModule,
      LayoutModule,
      MqttModule.forRoot(environment.MQTTCONFIG)
    ],
    providers: [],
    bootstrap: [AppComponent]
  })
  export class AppModule { }
  </code></pre>
  
  <p>I am getting the following error though:</p>
  
  <pre><code>ERROR in src/app/app.module.ts(62,24): error TS2345: Argument of type '{ broker: string; hostname: string; protocol: string; port: number; username: string; password: string; path: string; topic_query: string; topic_update_state: string; clientID: string; }' is not assignable to parameter of type 'IMqttServiceOptions'.
    Types of property 'protocol' are incompatible.
      Type 'string' is not assignable to type '"wss" | "ws"'.
  </code></pre>
  
  <p>Line 62 is this: MqttModule.forRoot(environment.MQTTCONFIG)</p>
  
  <p>Property protocol in the MQTT options is set to 'wss':
  <strong>protocol: 'wss'</strong></p>
  
  <p>How can I correct this error?</p>