MiOS: Only change Item state during incremental updates from MiOS Unit

When we detect a MiOS Unit has been restarted, we force a _Full refresh_ to see if anything has changed (as openHAB might be out of sync). During these full/forced refreshes, Item states _will only_ be updated in openHAB if they're different from the state currently in openHAB. This is done to keep things in sync that might have changed around the time of the MiOS Unit restart.

By using conditional Item state-changes Item during the Full refresh, we avoid unwanted Rule execution, as these would normally be re-triggered by the corresponding Item state change(s)

Move to using `UnDefType.NULL` for `Number` and `Date` Items, when the value comes in as the empty String.

Force the Character-Set on the MiOS JSON Response stream to UTF-8, for i18n, since MiOS doesn't apply it in their Response headers.

Add some [intentionally undocumented] parameters to allow for tweaks during field testing.

Fix setting the right value in newly added (MiosUnit) parameter, as identified by @watou during review of the original PR (# 1888)