Implement PSR7 HTTP message interfaces

[adri]

https://github.com/php-fig/fig-standards/blob/master/proposed/http-message.md

[adri]

When switching to Diactoros we might get HTTP string parsing for free https://github.com/zendframework/zend-diactoros/blob/master/src/Request/Serializer.php#L39 which is useful for #109.