filter: bugfix for FilterPresent
Merge fixes for ldap present filter encoding/decoding from Gregory Haik
filter : small code cleanup

Also use ber.DecodeString in DecompileFilter like with all other filters
filter_test : rewrite encode/decode tests to be table-driven

Only 1 test for now but other tests should be written, especially to check the various possible encoding issues...
cleanup : unexport FilterMap & remove FilterSubstringsMap (unneeded)