Fix Date Control Regression

Fix regression from b7fcb8b. Dates were disappearing completely until
the field was clicked into.

src/components/EditorWidgets/Date/DateControl.js

@@ -20,9 +20,14 @@ export default class DateControl extends React.Component {
     includeTime: PropTypes.bool,
   };
 
-  componentDidMount() {
-    const { includeTime, value, field, onChange } = this.props;
+  constructor(props) {
+    super(props);
+    const { field, includeTime } = this.props;
     this.format = field.get('format') || (includeTime ? DEFAULT_DATETIME_FORMAT : DEFAULT_DATE_FORMAT);
+  }
+
+  componentDidMount() {
+    const { value } = this.props;
 
     /**
      * Set the current date as default value if no default value is provided. An