public function __construct(MapInterface $headers)
{
if ((string) $headers->keyType() !== 'string' || (string) $headers->valueType() !== HeaderInterface::class) {
throw new InvalidArgumentException();
}
$this->headers = $headers->map(function (string $name, HeaderInterface $header) {
return new Pair((string) (new Str($name))->toLower(), $header);
});
}
