[1pt] Namespace overriding fails for `acl` vocab

[NSeydoux]

Starting from #220, the namespace overriding logic shifted from:

• On one hand, the overrideNamespace flag may be set to specify that the namespace inferred from the target vocabulary should be ignored, and that the provided namespace is the one that should be used. This fixes the issue with the acl vocabulary, where the namespace determination heuristics fails. In this case however, the provided 'overriding' namespace matches the actual namespace used in the vocabulary, it just fixes the heuristic
  to:
• On the other hand, it may be used to actually change the namespace as defined in the vocabulary to a different, desired namespace in the artifacts. This is the case of the ActivityStreams vocabulary, which defines all its IRIs against the non-standard http IRI instead of the https one. In this case, the heuristic for discovering the namespace works fine, and the override happens when generating the artifacts

This breaks the generation for some vocabularies, such as acl.