fix: return executeOperation for explain, if promise is desired

Author: mbroadst

lib/cursor.js

@@ -1020,8 +1020,7 @@ Cursor.prototype.transformStream = function(options) {
 Cursor.prototype.explain = function(callback) {
   if (this.operation) {
     this.operation.options.explain = true;
-    executeOperation(this.s.topology, this.operation, callback);
-    return;
+    return executeOperation(this.s.topology, this.operation, callback);
   }
 
   this.s.cmd.explain = true;