[bugfix] visualization flickers when rerunning query (apache#6374)

When hitting the Query button in the explore view, the previous chart
gets shown (flaskhed quickly) between the moment where the loading
spinner goes away and the new chart show up. This fix seems to prevent
this.

(cherry picked from commit e0b3927)

superset/assets/src/chart/Chart.jsx

@@ -79,7 +79,7 @@ class Chart extends React.PureComponent {
 
   handleRenderSuccess() {
     const { actions, chartStatus, chartId, vizType } = this.props;
-    if (chartStatus !== 'rendered') {
+    if (['loading', 'rendered'].indexOf(chartStatus) < 0) {
       actions.chartRenderingSucceeded(chartId);
     }