src/Http/Message.php
public function withAddedHeader($header, $value)
    {
        if (!$this->hasHeader($header)) {
            return $this->withHeader($header, $value);
        }
        $this->headers[strtolower($header)][] = $value;

        return $this;
    }
public function withAddedHeader($header, $value)
    {
        if (!$this->hasHeader($header)) {
            return $this->withHeader($header, $value);
        }
        $this->headers[strtolower($header)][] = $value;

        return $this;
    }
{@inheritdoc}