don't override document.title if set to None

This allows users to override `document.title` during runtime with a
clientside callback or a special component
(plotly/dash-core-components#833)

Author: chriddyp

dash-renderer/src/components/core/DocumentTitle.react.js

@@ -7,16 +7,13 @@ class DocumentTitle extends Component {
         super(props);
         const {update_title} = props.config;
         this.state = {
-            initialTitle: document.title,
             update_title,
         };
     }
 
     UNSAFE_componentWillReceiveProps(props) {
         if (this.state.update_title && props.isLoading) {
             document.title = this.state.update_title;
-        } else {
-            document.title = this.state.initialTitle;
         }
     }