Fix update error when PUTting new values: triples get lost

[vcharpenay]

(also reported by @remcollier)

With the current modeling of the RDF dataset, when turning a light on, it's possible to erase triples about the property itself. It shouldn't be the case. A modeling closer to SOSA would solve the problem.

Original resource

@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix sosa: <http://www.w3.org/ns/sosa/> .
@prefix ssn: <http://www.w3.org/ns/ssn/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

<http://127.0.1.1:8080/property-B3_42_GF_Z5_G8_LGHT_LOAD#it> rdf:type sosa:ActuableProperty ,
sosa:ObservableProperty ,
ssn:Property ;
rdf:value "off" ;
foaf:isPrimaryTopicOf <http://127.0.1.1:8080/property-B3_42_GF_Z5_G8_LGHT_LOAD> .

after a PUT request on the resource:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

<http://127.0.1.1:8080/property-B3_42_GF_Z5_G8_LGHT_LOAD#it> rdf:value "on" .