Return XPath integers as actual integers.

This is to ensure that calls such as string(10) return "10" and not "10.0". It
also saves integer -> float conversions when they're not needed.

lib/oga/xpath/evaluator.rb

@@ -794,7 +794,7 @@ module Oga
       # @return [Float]
       #
       def on_int(ast_node, context)
-        return ast_node.children[0].to_f
+        return ast_node.children[0]
       end
 
       ##