fix timeColumnType assignment

public/app/plugins/datasource/postgres/query_ctrl.ts

@@ -196,7 +196,7 @@ export class PostgresQueryCtrl extends QueryCtrl {
     this.target.timeColumn = this.timeColumnSegment.value;
     this.datasource.metricFindQuery(this.metaBuilder.buildDatatypeQuery(this.target.timeColumn)).then(result => {
       if (result.length === 1) {
-        this.target.timeColumnType = result[0];
+        this.target.timeColumnType = result[0].text;
       }
     });
     this.panelCtrl.refresh();