Traversal robustness

[DILewis]

1. In setting up a Caddy web server for RDF, I came across situations under which no Content-Type was being received, at which point, traversal breaks, because of string operations being performed on null. This patch resolves this. It also introduces a presumption of RDF nquads (which should, I think, fail gracefully if parsing doesn't work).
2. I've added a parameter to the traverser to permit a check for common file extensions – these shouldn't need checking for RDF, so can be excluded from traversal. I haven't benchmarked whether this improves performance.