Use Node#sameValueAs instead of Node#equals

[bnouwt]

According to Apache Jena's website we should not use the Node#equals(...) method to compare two nodes with each other, because this method does not take the real RDF semantics into account. Instead, we should use the Node#sameValueAs(...) method which does take RDF semantics into account.

In the current master branch, we use the equals(...) method in the following places:

Consider replacing these with the sameValueAs(...) method, but take into account whether this is actually what we want. Maybe also create some unit tests to discover whether this change actually makes a difference.