XPath function name typo silently ignored

If a function name is misspelled, REXML::XPath.match doesn't warn or raise.
```ruby
REXML::XPath.match doc, '//foo[positoin()=1]'
```

Weird case:
```ruby
doc = REXML::Document.new('<foo><namae>bar</namae><bar/></foo>')
REXML::XPath.match doc, '//*[namae()="bar"]'
#=> [<foo> ... </>]
```
