http: fix ParseURL to handle //relative_path properly

Fixes #900.

R=rsc
CC=golang-dev
https://golang.org/cl/1756042

src/pkg/http/url.go

@@ -318,7 +318,7 @@ func ParseURL(rawurl string) (url *URL, err os.Error) {
 	}
 
 	// Maybe path is //authority/path
-	if len(path) > 2 && path[0:2] == "//" {
+	if url.Scheme != "" && len(path) > 2 && path[0:2] == "//" {
 		url.Authority, path = split(path[2:], '/', false)
 	}
 	url.RawPath = path + query

src/pkg/http/url_test.go

@@ -174,6 +174,17 @@ var urltests = []URLTest{
 		},
 		"",
 	},
+	// leading // without scheme shouldn't create an authority
+	URLTest{
+		"//foo",
+		&URL{
+			Raw:     "//foo",
+			Scheme:  "",
+			RawPath: "//foo",
+			Path:    "//foo",
+		},
+		"",
+	},
 }
 
 var urlnofragtests = []URLTest{