net/http: fix bug parsing http_proxy lacking a protocol

Per the curl man page, the http_proxy configuration can be
of the form:

   [protocol://]<host>[:port]

And we had a test that <ip>:<port> worked, but if
the host began with a letter, url.Parse parsed the hostname
as the scheme instead, confusing ProxyFromEnvironment.

R=golang-dev
CC=golang-dev
https://golang.org/cl/6875060

src/pkg/net/http/transport.go

@@ -90,7 +90,7 @@ func ProxyFromEnvironment(req *Request) (*url.URL, error) {
 		return nil, nil
 	}
 	proxyURL, err := url.Parse(proxy)
-	if err != nil || proxyURL.Scheme == "" {
+	if err != nil || !strings.HasPrefix(proxyURL.Scheme, "http") {
 		if u, err := url.Parse("http://" + proxy); err == nil {
 			proxyURL = u
 			err = nil

src/pkg/net/http/transport_test.go

@@ -1068,6 +1068,9 @@ var proxyFromEnvTests = []struct {
 	wanterr error
 }{
 	{"127.0.0.1:8080", "http://127.0.0.1:8080", nil},
+	{"cache.corp.example.com:1234", "http://cache.corp.example.com:1234", nil},
+	{"cache.corp.example.com", "http://cache.corp.example.com", nil},
+	{"https://cache.corp.example.com", "https://cache.corp.example.com", nil},
 	{"http://127.0.0.1:8080", "http://127.0.0.1:8080", nil},
 	{"https://127.0.0.1:8080", "https://127.0.0.1:8080", nil},
 	{"", "<nil>", nil},