Backport the IntObjectHashMap changes in f23f3b9617b01095416334060ca8379316946e5c

Motivation:

The mentioned commit contains a bug fix and an improvement in
IntObjectHashMap that requires backporting.

Modifications:

Update IntObjectMap, IntObjectHashMap, and IntObjectHashMapTest

Result:

Easier to backport HTTP/2 and other changes in master in the future