relay: disable keep alive conns, they interact poorly with suspension

[skabbes]

I expect the DisableKeepAlives change to eliminate the EOF / conn reset errors. Without those errors we would never have found the re-read request body error, which is explicitly addressed by the new GetBody code.

A few justifications for the other changes:

1. remove all http.Transport timeouts. These are generally to prevent "bad" upstream servers, but I think up should try to be as invisible as possible, including not adding additional http timeouts. We do not know how people will use Up. Maybe they want to use it for delayed Continue responses.
2. remove specific GetBody code, this is already handled by http.NewRequest
3. Tests show new behavior for disabling keep-alives

[skabbes]

@tj @berzniz another promising change for the contentlength errors, TBH the DisableKeepAlives is probably the entire fix.

[skabbes]

Also, the reason the GetBody caused the 'no content' bug previously, was because internally http.NewRequest was type switching on the io.Reader passed as the body to set the ContentLength. ioutil.NopReader was hiding that. So, content length appeared to be 0 for all the requests.

[berzniz]

Thanks @skabbes 7 @tj! I'm running the new version and will report if it fixed the issues.