remove 'rawBody' request-attribute

[BreiteSeite]

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As @weierophinney mention in another #64 (comment), we don't really need the rawBody attribute on the request, as the body itself can always be retrieved via Request::getBody()

[weierophinney]

This repository has been closed and moved to mezzio/mezzio-helpers; a new issue has been opened at mezzio/mezzio-helpers#2.