FUJITSU: Improve handling of short (command only) messages. (#1784)

* add previous state support to `toCommon()`
Try to better keep the settings if we can when using the `IRac` class interface, if supplied a previous state. This better handles the short commands that don't have most of the settings in them.
Add unit tests to ensure it works as expected.
* Change `toString()` behaviour.
If we only have info from a _real_ short (command only) message, don't report other settings.
This makes text for single messages more accurate.
If the Class's state has been modified synthetically, report all the settings.

For #1780

src/IRac.cpp

@@ -4252,7 +4252,7 @@ namespace IRAcUtils {
       case decode_type_t::FUJITSU_AC: {
         IRFujitsuAC ac(kGpioUnused);
         ac.setRaw(decode->state, decode->bits / 8);
-        *result = ac.toCommon();
+        *result = ac.toCommon(prev);
         break;
       }
 #endif  // DECODE_FUJITSU_AC