Implement PSR-7 http-message interfaces

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https://github.com/php-fig/http-message

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For the reference: sabre/http has explicitly chosen to not adhere to the complete interface. Obviously, this needs a review.

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For the reference: I'm currently looking into this. In particular how this works out while considering #30 in order to implement #46.

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Closed via #54.