fix: preserve aggregate explain support for legacy servers

Author: mbroadst

lib/cursor.js

@@ -1019,7 +1019,7 @@ Cursor.prototype.transformStream = function(options) {
  */
 Cursor.prototype.explain = function(callback) {
   if (this.operation) {
-    this.operation.explain = true;
+    this.operation.options.explain = true;
     executeOperation(this.s.topology, this.operation, callback);
     return;
   }

lib/operations/aggregate.js

@@ -96,6 +96,7 @@ class AggregateOperation extends CommandOperationV2 {
     }
 
     if (options.explain) {
+      options.full = false;
       command.explain = options.explain;
     }
 

lib/operations/command_v2.js

@@ -29,11 +29,6 @@ class CommandOperationV2 extends OperationBase {
     }
 
     let fullResponse = this.options.full;
-    if (this.explain) {
-      cmd = { explain: cmd };
-      fullResponse = false;
-    }
-
     server.command(this.ns.toString(), cmd, this.options, (err, result) => {
       if (err) {
         callback(err, null);