Problem with Wifi persistent connection

### Basic Infos

- [x] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [x] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [ ] I have tested that the issue is present in current master branch (aka latest git).
- [x] I have searched the issue tracker for a similar issue.
- [ ] If there is a stack dump, I have decoded it.
- [x] I have filled out all fields below.

#### Platform

- Hardware: [ESP-07]
- Core Version: [3.1.1/NONOSDK22x_191122 ]
- Development Env: [Arduino IDE]
- Operating System: [Windows]

### Settings in IDE

- Module: [Generic ESP8266 Module]
- Flash Mode: [dout]
- Flash Size: [1MB]
- lwip Variant: [v2 Lower Memory]
- Reset Method: [dtr]
- Flash Frequency: [40Mhz]
- CPU Frequency: [80Mhz]
- Upload Using: [SERIAL]
- Upload Speed: [115200)

### Problem Description
Wifi persistent connection option did not work for me, each reconnection took a long time (4500 ms).
I started debugging and found that threshold property does not preserved when station config saved to flash then loaded.

### Sketch

```cpp
#include "ESP8266WiFi.h"

void setup() {
Serial.begin(115200);

struct station_config conf;

strcpy((char*)conf.ssid, "testAP");
strcpy((char*)conf.password, "password");
conf.bssid_set = 0;
conf.threshold.authmode = AUTH_WPA_PSK;
conf.threshold.rssi = -127;

WiFi.persistent(true);
WiFi.mode(WIFI_STA);
int res = wifi_station_set_config(&conf);

memset(&conf, 0, sizeof(station_config));

wifi_station_get_config_default(&conf);

Serial.print("\r\n");
Serial.printf(PSTR(" ssid: %s\n"), conf.ssid);
Serial.printf(PSTR(" password: %s\n"), conf.password);
Serial.printf(PSTR(" bssid_set: %d\n"), conf.bssid_set);
Serial.printf(PSTR(" bssid: %02x:%02x:%02x:%02x:%02x\n"),
conf.bssid[0], conf.bssid[1], conf.bssid[2],
conf.bssid[3], conf.bssid[4], conf.bssid[5]);
Serial.printf(PSTR(" threshold.rssi: %d\n"), conf.threshold.rssi);
Serial.printf(PSTR(" threshold.authmode: %d\n"), (uint8_t)conf.threshold.authmode);

}

void loop() {
// put your main code here, to run repeatedly:

}
```

### Debug Messages

```
ssid: testAP
password: password
bssid_set: 0
bssid: 21:66:d8:15:8c
threshold.rssi: 0
threshold.authmode: 0
```

For this reason next code always return false
https://github.com/esp8266/Arduino/blob/badb407bbb7f6dec18d4dd7fd83785b88ec7e6f2/libraries/ESP8266WiFi/src/ESP8266WiFiSTA.cpp#L89-L95

I commented out this lines, and it worked as it should, reconnection become much faster (700ms).
Is it bug, SDK issue, or my environment settings problem?