Revert "don't revert document.title"

This reverts commit 6e97225.

Author: chriddyp

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

@@ -7,13 +7,16 @@ 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;
         }
     }