Set the message headers.
See http\Message::getHeaders() and http\Message::addHeaders().
-> **NOTE:**
+> ***NOTE:***
> Prior to v2.5.6/v3.1.0 headers with the same name were merged into a single
> header with values concatenated by comma.
## Changelog:
-Version | Changes
--------------|--------
-2.5.6, 3.1.0 | Multiple headers with the same name are kept separate instead of merged together.
+0. 2.5.6, 3.1.0
+ * Multiple headers with the same name are kept separate instead of merged together.