[web] Fix HEAD request handling (closes #3677949) Don't special-case HEAD in CubicWebRequestBase.is_client_cache_valid so we don't return garbage. HEAD is supposed to return the actual headers that a GET would return, which we can't do if we bypass all the normal cubicweb logic. In particular, we no longer always return 200, and return the right Content-Length/Content-Type/...

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