[Patch] setDateTime and Captive portal can set invalid date

The rtcDateTime class has no checking that the entered date is valid using string date or int date time values. The aRest setDateTime function just check if date and time strings are provided which could be anything. Combine the two and you get buffer overflow when bad values are used and if buffer does not overflow then you get ridiculous date/time being set. Same goes for Captive portal.

I have added RtcDateTime.isValid() to test validity and updated aREST setDateTime & Captive Portal to use it. See:

github.com/gb0101010101/SDL_ESP8266_WeatherPlus/commit/418b086831a3073eeabe0b4d4ab1813a7baf175a