XPath underscored function name is wrongly accepted


```ruby
# local-name: OK
REXML::XPath.match REXML::Document.new('<foo/>'), 'local-name(*)'
#=> ["foo"]

# local_name: Should be rejected
REXML::XPath.match REXML::Document.new('<foo/>'), 'local_name(*)'
#=> ["foo"]
```
