Fixed CSS evaluation of :first-of-type

The old XPath "position() = 1" would work in Nokogiri due to the way they
retrieve descendants. In Oga however this would simply always return the first
node.

To fix this Oga now counts the amount of preceding siblings that match the same
full name.

lib/oga/css/parser.y

@@ -515,7 +515,11 @@ end
   # @return [AST::Node]
   #
   def on_pseudo_class_first_of_type
-    return s(:eq, s(:call, 'position'), s(:int, 1))
+    return s(
+      :eq,
+      s(:call, 'count', s(:axis, 'preceding-sibling', s(:call, 'name'))),
+      s(:int, 0)
+    )
   end
 
   ##