internal/frontend: handle file paths with spaces

http.Handle generally handles the escaping for us, except for the
first space which is used to split out the method.

Fixes golang/go#73534

Change-Id: I588292ce5ffa6228f57c8e2c5f0ac5e73c27b853
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/668875
Reviewed-by: Jonathan Amsterdam <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
kokoro-CI: kokoro <[email protected]>

Author: seankhliao

internal/frontend/frontend_test.go

@@ -131,10 +131,10 @@ func TestTagRoute(t *testing.T) {
 
 func TestInstallFS(t *testing.T) {
 	s, handler := newTestServer(t, nil)
-	s.InstallFS("/dir", os.DirFS("."))
+	s.InstallFS("/a dir", os.DirFS("."))
 	// Request this file.
 	w := httptest.NewRecorder()
-	handler.ServeHTTP(w, httptest.NewRequest("GET", "/files/dir/frontend_test.go", nil))
+	handler.ServeHTTP(w, httptest.NewRequest("GET", "/files/a%20dir/frontend_test.go", nil))
 	if w.Code != http.StatusOK {
 		t.Errorf("got status code = %d, want %d", w.Code, http.StatusOK)
 	}

internal/frontend/server.go

@@ -307,7 +307,7 @@ func (s *Server) installDebugHandlers(handle func(string, http.Handler)) {
 
 // InstallFS adds path under the /files handler, serving the files in fsys.
 func (s *Server) InstallFS(path string, fsys fs.FS) {
-	s.fileMux.Handle(path+"/", http.StripPrefix(path, http.FileServer(http.FS(fsys))))
+	s.fileMux.Handle("GET "+path+"/", http.StripPrefix(path, http.FileServer(http.FS(fsys))))
 }
 
 const (