system/HTTP/Message.php
public function appendHeader(string $name, string $value)
	{
		$orig_name = $this->getHeaderName($name);

		$this->headers[$orig_name]->appendValue($value);

		return $this;
	}
public function appendHeader(string $name, string $value)
	{
		$orig_name = $this->getHeaderName($name);

		$this->headers[$orig_name]->appendValue($value);

		return $this;
	}
Adds an additional header value to any headers that accept
multiple values (i.e. are an array or implement ArrayAccess)

@param string $name
@param string $value

@return Message