Fixed regression introduced in edaaee283e3bbfd9af36dec3eaf67452472a0448

The correct function in Network.URI is `isURI`, which is documented
thus:

> "Test if string contains a valid URI (an absolute URI with optional
fragment identifier)."

RDF URIs are optionally appended with fragments e.g. `#foo`. The
edaaee283 commit incorrectly opted for `isAbsoluteURI`, which is not
the same as RDF URIs given the following haddock documentation:

> "Test if string contains a valid absolute URI (an absolute URI without
a fragment identifier)."

Also, this identified a new problem in the turtle parser tests from
`data/ttl/example7`, which includes "ns10:Édimbourg" in the parsing
test.

URIs conform to RFC3986 which does not support unicode, i.e. the É
symbol. IRIs conform to RFC3987 which includes unicode. The rdf4h
library does not include IRIs, only URIs, hence this unit test has
been disabled. See:

https://tools.ietf.org/html/rfc3986
https://tools.ietf.org/html/rfc3987