QContactManagerEngine::CompareVariant() does not support QVariant::Url

QContactManagerEngine::CompareVariant() is used by QContactManagerEngine::testFilter() to do variant comparison when the MatchExactly flag is used.
The issue is that QContactManagerEngine::CompareVariant() does not support comparison of QVariant::Url type and will return 0 (which is considered as a "match" by QContactManagerEngine::testFilter().

As a consequence, making the following filter would match ALL contacts (if testFilter() is used):
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QContactDetailFilter filter;
filter.setDetailDefinitionName(QContactUrl::DefinitionName, QContactUrl::FieldUrl);
filter.setMatchFlags(QContactFilter::MatchExactly);
filter.setValue("http://mydummyurl.fi");
ret = manager->contacts(filter);
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