fix: net/http transport goroutines leak

[laoleesch]

Hi! For each transport flush there hung a couple "zombie"-routines
net/http.(*persistConn).readLoop
net/http.(*persistConn).writeLoop

I suppose it because there is no response.Body.Close() in transport cycles. This not only leads to a zombie-routines but also because of the lack of re-using connections to the loss of performance (no keep-alive).

net/http/#Client.Do

If the returned error is nil, the Response will contain a non-nil Body which the user is expected to close. If the Body is not both read to EOF and closed, the Client's underlying RoundTripper (typically Transport) may not be able to re-use a persistent TCP connection to the server for a subsequent "keep-alive" request.

It solves resource leak issue in my cases.

Thanks!

[laoleesch]

Issue #316

[rhcarvalho]

I think we also need to drain the response body, could use io.CopyN (see google/go-github#317 (comment)).
Not sure this is still relevant in recent Go versions, the docs suggest so.

https://golang.org/pkg/net/http/#Client.Do

If the returned error is nil, the Response will contain a non-nil Body which the user is expected to close. If the Body is not both read to EOF and closed, the Client's underlying RoundTripper (typically Transport) may not be able to re-use a persistent TCP connection to the server for a subsequent "keep-alive" request.

https://golang.org/pkg/net/http/#Response

// [...] It is the caller's responsibility to
// close Body. The default HTTP client's Transport may not
// reuse HTTP/1.x "keep-alive" TCP connections if the Body is
// not read to completion and closed.