src/Message.php
private function addHeader($name, $value)
	{
		if (!$this->hasHeader($name)) {
			return $this->setHeader($name, $value);
		}

		if (!is_array($value)) {
			$value = [$value];
		}

		foreach ($value as $element) {
			$this->headers[$this->headerNames[strtolower($name)]][] = $element;
		}

		return $this;
	}
private function addHeader($name, $value)
	{
		if (!$this->hasHeader($name)) {
			return $this->setHeader($name, $value);
		}

		if (!is_array($value)) {
			$value = [$value];
		}

		foreach ($value as $element) {
			$this->headers[$this->headerNames[strtolower($name)]][] = $element;
		}

		return $this;
	}
Add the header.

@param string $name
@param string|string[] $value
@return $this