Handle exception if body is already parsed

[nathanclevenger]

If CTX.do is called twice, and there is a body, we need to more elegantly handle the situation. Maybe we want to default to cloning the request so the body stream isn't read and destroyed.

[nathanclevenger]

This is definitely an issue if we're passing the request into any other service binding or durable object, so we have to resolve this one first.

[sam-lippert]

#38