refactor: couple of minor improvements

Author: vkarpov15

lib/query.js

@@ -2361,10 +2361,10 @@ Query.prototype._find = async function _find() {
 
   // Separate options to pass down to `completeMany()` in case we need to
   // set a session on the document
-  const completeManyOptions = Object.assign({}, {
+  const completeManyOptions = {
     session: this && this.options && this.options.session || null,
     lean: mongooseOptions.lean || null
-  });
+  };
 
   const options = this._optionsForExec();
 
@@ -2382,7 +2382,7 @@ Query.prototype._find = async function _find() {
   }
 
   if (!mongooseOptions.populate) {
-    const versionKey = _this.schema.options.versionKey;
+    const versionKey = this.schema.options.versionKey;
     if (mongooseOptions.lean && mongooseOptions.lean.versionKey === false && versionKey) {
       docs.forEach((doc) => {
         if (versionKey in doc) {