Fix for httpbakery.Do function

When a POST request is issued, we need to replay the body contet
for the repeated request with discharged macaroons.
I suggest we just pass the string into ctxt.post, and inside there, we can do:

      getBody := func() io.ReadCloser {
            return ioutil.NopCloser(strings.NewReader(body))
      }
      ....
      return ctxt.do(req, getBody)

Given ioutil.NopCloser, the bodyReader type is unnecessary.