set default value of httpMethod

public/app/plugins/datasource/prometheus/config_ctrl.ts

+export class PrometheusConfigCtrl {
+  static templateUrl = 'public/app/plugins/datasource/prometheus/partials/config.html';
+  current: any;
+
+  /** @ngInject */
+  constructor($scope) {
+    this.current.jsonData.httpMethod = this.current.jsonData.httpMethod || 'GET';
+  }
+}

public/app/plugins/datasource/prometheus/datasource.ts

@@ -34,7 +34,7 @@ export class PrometheusDatasource {
     this.basicAuth = instanceSettings.basicAuth;
     this.withCredentials = instanceSettings.withCredentials;
     this.interval = instanceSettings.jsonData.timeInterval || '15s';
-    this.httpMethod = instanceSettings.jsonData.httpMethod || 'GET';
+    this.httpMethod = instanceSettings.jsonData.httpMethod;
   }
 
   _request(method, url, data?, requestId?) {

public/app/plugins/datasource/prometheus/module.ts

 import { PrometheusDatasource } from './datasource';
 import { PrometheusQueryCtrl } from './query_ctrl';
-
-class PrometheusConfigCtrl {
-  static templateUrl = 'partials/config.html';
-}
+import { PrometheusConfigCtrl } from './config_ctrl';
 
 class PrometheusAnnotationsQueryCtrl {
   static templateUrl = 'partials/annotations.editor.html';