#25 Fix bug #13

[Wim Nelis]

This patch is a solution to bug #13 (see https://sourceforge.net/p/devmon/bugs/13/ ).

Module dm_tests, function oid_hash is modified to always load the SNMP OIDs in hash %oids, thus reloading any definitions of the exceptions and the thresholds for OIDS already cached. Function tests in the same module is modified to redo the threshold checks in case the value of an OID is already known / cached.