src/Http/Message.php
public function withHeader($header, $value)
    {
        $header = trim($header);
        $name = strtolower($header);
        if (!is_array($value)) {
            $this->headers[$name] = array(trim($value));
        } else {
            $this->headers[$name] = array_map('trim', $value);
        }

        return $this;
    }
public function withHeader($header, $value)
    {
        $header = trim($header);
        $name = strtolower($header);
        if (!is_array($value)) {
            $this->headers[$name] = array(trim($value));
        } else {
            $this->headers[$name] = array_map('trim', $value);
        }

        return $this;
    }
{@inheritdoc}