Fixed AST for :only-of-type

lib/oga/css/parser.y

@@ -581,7 +581,7 @@ end
   # @return [AST::Node]
   #
   def on_pseudo_class_only_of_type
-    return s(:eq, s(:call, 'last'), s(:int, 1))
+    return s(:and, on_pseudo_class_first_of_type, on_pseudo_class_last_of_type)
   end
 
   ##

spec/oga/css/parser/pseudo_classes/only_of_type_spec.rb

@@ -4,7 +4,15 @@ describe Oga::CSS::Parser do
   context ':only-of-type pseudo class' do
     example 'parse the :only-of-type pseudo class' do
       parse_css(':only-of-type').should == parse_xpath(
-        'descendant::*[last() = 1]'
+        'descendant::*[count(preceding-sibling::*) = 0 and ' \
+          'count(following-sibling::*) = 0]'
+      )
+    end
+
+    example 'parse the a:only-of-type pseudo class' do
+      parse_css('a:only-of-type').should == parse_xpath(
+        'descendant::a[count(preceding-sibling::a) = 0 and ' \
+          'count(following-sibling::a) = 0]'
       )
     end
   end