net/url: use EscapedPath for url.JoinPath

Fixes golang#53763

Change-Id: I08b53f159ebdce7907e8cc17316fd0c982363239
Reviewed-on: https://go-review.googlesource.com/c/go/+/416774
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: seankhliao

src/net/url/url.go

@@ -1193,7 +1193,7 @@ func (u *URL) UnmarshalBinary(text []byte) error {
 func (u *URL) JoinPath(elem ...string) *URL {
 	url := *u
 	if len(elem) > 0 {
-		elem = append([]string{u.Path}, elem...)
+		elem = append([]string{u.EscapedPath()}, elem...)
 		p := path.Join(elem...)
 		// path.Join will remove any trailing slashes.
 		// Preserve at least one.

src/net/url/url_test.go

@@ -2119,6 +2119,16 @@ func TestJoinPath(t *testing.T) {
 			elem: nil,
 			out:  "https://go.googlesource.com/",
 		},
+		{
+			base: "https://go.googlesource.com/a%2fb",
+			elem: []string{"c"},
+			out:  "https://go.googlesource.com/a%2fb/c",
+		},
+		{
+			base: "https://go.googlesource.com/a%2fb",
+			elem: []string{"c%2fd"},
+			out:  "https://go.googlesource.com/a%2fb/c%2fd",
+		},
 		{
 			base: "/",
 			elem: nil,