Update CHANGELOG.md with PR 1594.

Author: James McCarthy

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 #### Features
 
+* [#1594](https://github.com/ruby-grape/grape/pull/1594): Make Hashie::Mash params optional - [@james2m](https://github.com/james2m)
 * [#1555](https://github.com/ruby-grape/grape/pull/1555): Added code coverage w/Coveralls - [@dblock](https://github.com/dblock).
 * [#1568](https://github.com/ruby-grape/grape/pull/1568): Add `proc` option to `values` validator to allow custom checks - [@jlfaber](https://github.com/jlfaber).
 * [#1575](https://github.com/ruby-grape/grape/pull/1575): Include nil values for missing nested params in declared - [@thogg4](https://github.com/thogg4).