Set the message headers.
See http\Message::getHeaders() and http\Message::addHeaders().
+> **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.
+
## Params:
* array $headers = NULL
array(0) {
}
+## Changelog:
+
+Version | Changes
+-------------|--------
+2.5.6, 3.1.0 | Multiple headers with the same name are kept separate instead of merged together.