Allow sending domain-specific resources to servers

All test cases that send (POST or PUT) a resource to the server always use the same hard-coded set of triples (Bug model). This implies that the test suite can only be applied to servers without domain restrictions. 

This modification facilitates customization of this dummy resource in order to avoid errors that actually are not. In case it is necessary to specify the resource representation to be used, it would be nice to provide a way to link it to the Test Suite so that it can be deserialized within the postContent() method (LDPTest class). If this resource is not provided, the TS could still use the hard-coded resource.