Include the body of the request in the UnhandledHTTPRequestError when matching on the body

If either the current cassette or the default cassette options include the body as a matcher, show the body of the request in the UnhandledHTTPRequestError to help identify the unhandled request.

I've been working with the FreshBooks API recently, which uses a single URI for all requests and pushes the parameters of the request into the body, so showing just the method/URI when there's an unhandled request is of little help in determining what request is responsible for the error.