Update handlers skipped empty strings and nulls, so a client could not
blank or null out a column. Every key present in the payload now
overwrites the stored value, including "" and null.
- add common.MergeUpdateValues and use it in resolvespec and restheadspec
- add Handler.SetDisallowNulls to skip null values (""still overwrites)
- websocketspec and mqttspec now write only the keys present in the
payload via SetMap instead of updating the whole zeroed model, which
clobbered absent fields
