Bump copyright

Author: Starcounter-Jack

src/duplex.ts

@@ -1,6 +1,6 @@
 /*!
  * https://github.com/Starcounter-Jack/JSON-Patch
- * (c) 2017 Joachim Wester
+ * (c) 2017-2021 Joachim Wester
  * MIT license
  */
 import { _deepClone, _objectKeys, escapePathComponent, hasOwnProperty } from './helpers.js';
@@ -209,4 +209,4 @@ export function compare(tree1: Object | Array<any>, tree2: Object | Array<any>,
   var patches = [];
   _generate(tree1, tree2, patches, '', invertible);
   return patches;
-}
+}