[elasticsearch] Fix bug when switching from "Raw Document" metric type

when switch to "raw Document" metric type we do free all "Group by"
however when we switch back to another type we do not reset the default aggregation (date histogram)
Thus all modification will through exception as no "Group by" is defined and panel should be recreated
the fix will reintialize the "Group by" by setting default value

public/app/plugins/datasource/elasticsearch/query_builder.js

@@ -194,12 +194,13 @@ function (queryDef) {
     if (target.bucketAggs.length === 0) {
       metric = target.metrics[0];
       if (metric && metric.type !== 'raw_document') {
-        throw {message: 'Invalid query'};
-      }
+        target.bucketAggs = [{type: 'date_histogram', id: '2', settings: {interval: 'auto'}}];
+      } else {
         var size = metric && metric.hasOwnProperty("settings") && metric.settings.hasOwnProperty("size")
                    && metric.settings["size"] !== null ? metric.settings["size"] : 500 ;
         return this.documentQuery(query,size);
       }
+    }
 
     nestedAggs = query;