src/Message/Message.php
protected function headerKey($name)
    {
        $key = $name;
        foreach (array_keys($this->headers) as $existingName) {
            if (strtolower($name) == strtolower($existingName)) {
                $key = $existingName;
                break;
            }
        }
        return $key;
    }
protected function headerKey($name)
    {
        $key = $name;
        foreach (array_keys($this->headers) as $existingName) {
            if (strtolower($name) == strtolower($existingName)) {
                $key = $existingName;
                break;
            }
        }
        return $key;
    }
Get the original (preserved) header name for provided name

@param $name
@return mixed