fix forgotten case in last commit

fixes #6
Add the SQL function: md5
Merge pull request #7 from jpdeplaix/md5

Add the SQL function: md5
Add the type for string arrays
Add the array_agg aggregator
Provide a polymorphic type to array_agg
Fix the internal type of array_agg and add a generic TArray type
Add all the other types of array handled by PGOCaml