Revert "rewrote RDF serialization to use a much simplified Turtle format"

after internal discussions, we concluded that rather than crippling the
RDF representations, a simplified response format should be provided via
a separate representation

This reverts commit e5c7465efc95a2b8ca84b29a2055514dd22a9810.
improved test coverage
added plain text representation

cf. e5c7465efc95a2b8ca84b29a2055514dd22a9810 and
49ec8bbbdd70d080b10508d25213dfb5c57ea357 for rationale