Fix attr2rdf/to_rdf

[franzlst]

The logic for separating the prefix from the predicate was to simple. Also my first attempt in fixing this (see 74b0322e4bef8b15e5972085cbe71d81345520eb) was not sufficient, as both the prefix and the predicate can potentially contain underscores. Therefore, we need to iterate over all underscores to see which one is used as separator.