Reel::RequestBody

Encapsulate request bodies in their own object. This allows easy conversion to a
string in cases where streaming isn't desired, or streaming as an IO-alike
through #read and #readpartial

This is a breaking change from the previous API. People who were using
request.body will now need to use request.body.to_s. The previous callback-based
API for reading the body is completely removed.

/cc @adstage-david @raggi @halorgium