fix truncation of unknown setting ID in SettingID.String

http2.go

@@ -138,7 +138,7 @@ func (s SettingID) String() string {
 	if v, ok := settingName[s]; ok {
 		return v
 	}
-	return fmt.Sprintf("UNKNOWN_SETTING_%d", uint8(s))
+	return fmt.Sprintf("UNKNOWN_SETTING_%d", uint16(s))
 }
 
 func validHeader(v string) bool {

http2_test.go

@@ -40,6 +40,7 @@ func TestSettingString(t *testing.T) {
 		want string
 	}{
 		{Setting{SettingMaxFrameSize, 123}, "[MAX_FRAME_SIZE = 123]"},
+		{Setting{1<<16 - 1, 123}, "[UNKNOWN_SETTING_65535 = 123]"},
 	}
 	for i, tt := range tests {
 		got := fmt.Sprint(tt.s)