Set a single header.
See http\Message::getHeader() and http\Message::addHeader().
Prior to v2.5.6/v3.1.0 headers with the same name were merged into a single
header with values concatenated by comma.
- string $header
The header’s name.
- Optional mixed $value = NULL
The header’s value. Removes the header if NULL.
- v2.5.6, v3.1.0
- Multiple headers with the same name are kept separate instead of merged together.