Comment 7 for bug 1194763

IRC summary: while this is valid XPath syntax, it's not a wanted feature in the context of xpathselect. The intention is that select_many() never asks for all nodes, but always specifies attribute filters. So we need to teach it to stringify the value instead of limiting attributes to string values, and also drop the curious limitation of not allowing '_' in property names. So the current bug title is quite appropriate.