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

Change values only during incremental updates from MiOS Unit, to avoid Rule execution upon MiOS restart.
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.
Fix setting the right value in newly added (MiosUnit) parameter, as identified by @watou during review.
Merge pull request #1889 from mrguessed/mios-patch-refresh

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