fromClone function for ecdh symkey

src/type/ecdh_symkey.js

@@ -63,3 +63,7 @@ ECDHSymmetricKey.prototype.read = function (input) {
 ECDHSymmetricKey.prototype.write = function () {
   return util.concatUint8Array([new Uint8Array([this.data.length]), this.data]);
 };
+
+ECDHSymmetricKey.fromClone = function (clone) {
+  return new ECDHSymmetricKey(clone.data);
+};