net/http: fix confusing shadowing in ProxyFromEnvironment

The old code worked, somewhat on accident, but was confusing,
and had a useless assignment to the inner err. It worked
because url.Parse parses just about anything, so the outer err
was always nil, so it always fell through to the bottom return
statement, even without the "err = nil" line.

Instead, just have two return statements, and add a comment.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/10448044

src/pkg/net/http/transport.go

@@ -109,9 +109,11 @@ func ProxyFromEnvironment(req *Request) (*url.URL, error) {
 	}
 	proxyURL, err := url.Parse(proxy)
 	if err != nil || !strings.HasPrefix(proxyURL.Scheme, "http") {
-		if u, err := url.Parse("http://" + proxy); err == nil {
-			proxyURL = u
-			err = nil
+		// proxy was bogus. Try prepending "http://" to it and
+		// see if that parses correctly. If not, we fall
+		// through and complain about the original one.
+		if proxyURL, err := url.Parse("http://" + proxy); err == nil {
+			return proxyURL, nil
 		}
 	}
 	if err != nil {