Updated changelog

Author: Marcel Overdijk

Changes.md

@@ -18,6 +18,8 @@ This update focuses on new features and bug fixes.
     For the `JavaScriptHandler` no external dependency is needed. (#32, #36)
   * Fix for `removeHandler()` causing a `ConcurrentModificationException`.
   * Fix for making wildcards in optionals not matchable (#42).
+  * Add static constants for custom error message keys.
+  * Add static constant for `"undefined"` string (#41).
   * Reorganize tests in proper separation of unit and integration tests (#38).
 
 ## v0.8.1 - February 20, 2017