Remove stale keys in OrderedProperties compute and merge

[rootvector2]

OrderedProperties keeps a parallel orderedKeys set that backs keys(), keySet(), entrySet() and toString(), but compute() and merge() only ever add to it. both operations remove the mapping from the backing Hashtable when the remapping function returns null, so after compute(key, (k, v) -> null) on a present key the mapping is gone yet the key still shows up in keySet(), size() disagrees with the key view, and toString() throws NullPointerException on the stale key's null value.

remove the key from orderedKeys when the result is null. a Hashtable never stores null values, so a null result means the mapping was removed, which is what the existing remove() overrides already do. found while auditing the ordered-view bookkeeping against the Properties contract; SortedProperties derives its keys live from super.keySet() and needs no change.