Test driver against non-string literals

[agarciadom]

At the moment, all the objects in our statements have been either RDF resources, or string literals. We should try creating a Turtle file with non-string literals (for instance, xs:integer literals) and seeing if that raises any issues in the driver.

We should also have tests checking that non-string literals should not go through the filtering of the language preferences (we may have to tweak the README accordingly as well).