src/Message.php
public function withHeader($name, $value)
    {
        if ( ! is_array($value))
        {
            $value = [$value];
        }

        $this->_headers[$name] = $value;
        return $this;
    }
public function withHeader($name, $value)
    {
        if ( ! is_array($value))
        {
            $value = [$value];
        }

        $this->_headers[$name] = $value;
        return $this;
    }
{@inheritdoc}