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.
done.
> On 12 Nov 2014, at 13:19, Roger Peppe <notifications@github.com> wrote:
> 
> In httpbakery/client.go:
> 
> >  	log.Printf("} -> error %#v", err)
> >  	return resp, err
> >  }
> >  
> > -func (ctxt *clientContext) do1(req *http.Request) (*http.Response, error) {
> > +func (ctxt *clientContext) do1(req *http.Request, getBody func() io.ReadCloser) (*http.Response, error) {
> > +	if getBody != nil {
> > +		req.Body = getBody()
> > +	}
> I think we should do:
> 
> else {
>       req.Body = nil
> }
> otherwise people are going to pass in the body in req.Body anyway, and will be surprised when it doesn't work.
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/rogpeppe/macaroon/pull/27/files#r20215303>.
>