Fix data race in MITM'ed HTTPS request handling

This data race requires quite of few conditions to appear.

1) The transport has to have HTTP/2 enabled (in goproxy, it's not by default).
2) The target server must be an HTTP/2 one.
3) The request must have a body.
4) The server must return certain erroneus HTTP status code.
5) The client must ignore Connection: close header from the proxy
   (4ec7933).

Somehow, I managed to hit all these conditions, and got this error:

    panic: runtime error: slice bounds out of range [96:48]

    goroutine 341699 [running]:
    bufio.(*Reader).ReadSlice(0xc000b0c1e0, 0x80?)
        /usr/local/go/src/bufio/bufio.go:347 +0x225
    bufio.(*Reader).ReadLine(0xc000b0c1e0)
        /usr/local/go/src/bufio/bufio.go:401 +0x27
    net/textproto.(*Reader).readLineSlice(0xc0013400f0)
        /usr/local/go/src/net/textproto/reader.go:56 +0x99
    net/textproto.(*Reader).ReadLine(...)
        /usr/local/go/src/net/textproto/reader.go:39
    net/http.readRequest(0xc000847d40?)
        /usr/local/go/src/net/http/request.go:1042 +0xba
    net/http.ReadRequest(0xc000b0c1e0?)
        /usr/local/go/src/net/http/request.go:1025 +0x19
    github.com/elazarl/goproxy.(*ProxyHttpServer).handleHttps.func2()
        /root/go/pkg/mod/github.com/elazarl/goproxy@v0.0.0-20220901064549-fbd10ff4f5a1/https.go:221 +0x3db
    created by github.com/elazarl/goproxy.(*ProxyHttpServer).handleHttps
        /root/go/pkg/mod/github.com/elazarl/goproxy@v0.0.0-20220901064549-fbd10ff4f5a1/https.go:211 +0x611

Despite 5), goproxy should not break from non-compliant HTTP clients,
and "Connection: close" is more like of an accidental fix.

See also golang/go#61596 (comment)

https.go

@@ -250,7 +250,12 @@ func (proxy *ProxyHttpServer) handleHttps(w http.ResponseWriter, r *http.Request
 						return
 					}
 					removeProxyHeaders(ctx, req)
-					resp, err = ctx.RoundTrip(req)
+					resp, err = func() (*http.Response, error) {
+						// explicitly discard request body to avoid data races in certain RoundTripper implementations
+						// see https://github.com/golang/go/issues/61596#issuecomment-1652345131
+						defer req.Body.Close()
+						return ctx.RoundTrip(req)
+					}()
 					if err != nil {
 						ctx.Warnf("Cannot read TLS response from mitm'd server %v", err)
 						return