Fix double slashes in POST request to ES (#5536)

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

@@ -256,7 +256,7 @@ function (angular, _, moment, kbn, ElasticQueryBuilder, IndexPattern, ElasticRes
       esQuery = esQuery.replace(/\$timeTo/g, range.to.valueOf());
       esQuery = header + '\n' + esQuery + '\n';
 
-      return this._post('/_msearch?search_type=count', esQuery).then(function(res) {
+      return this._post('_msearch?search_type=count', esQuery).then(function(res) {
         var buckets = res.responses[0].aggregations["1"].buckets;
         return _.map(buckets, function(bucket) {
           return {text: bucket.key, value: bucket.key};