Adds CHANGELOG entry for #204

Author: michalbundyra

CHANGELOG.md

@@ -22,7 +22,7 @@ All notable changes to this project will be documented in this file, in reverse
 
 ### Fixed
 
-- Nothing.
+- [#204](https://github.com/zendframework/zend-http/pull/204) fixes numerous header classes to cast field value to string (since `HeaderInterface::getFieldValue()` specifies a return value of a string).
 
 ## 2.11.0 - 2019-12-03