MapVariableResolverFactory doesn't check next factory for known variables

[jdimeo]

This method seems to have a bug in the implementation and is doing the exact same thing regardless of the presence of a next factory. Shouldn't the first one recursively check the next factory if this factory doesn't know about the variable?
https://github.com/mvel/mvel/blob/master/src/main/java/org/mvel2/integration/impl/MapVariableResolverFactory.java#L119

This actually seems to be overridding a working implementation in BaseVariableResolverFactory. Maybe we can just remove the subclass's override method entirely.

I'm working around this by making a subclass with this overridden implementation:

@Override
public Set<String> getKnownVariables() {
	var ret = super.getKnownVariables();
	if (getNextFactory() != null) {
		ret.addAll(getNextFactory().getKnownVariables());
	}
	return ret;
}