Merge the CSS "expression" and "path" parser rules

lib/oga/css/parser.y

@@ -7,16 +7,10 @@ options no_result_var
 
 rule
   css
-    : expression { val[0] }
+    : path       { val[0] }
     | /* none */ { nil }
     ;
 
-  expression
-    : path
-    | node_test
-    | axis
-    ;
-
   path_member
     : node_test
     | axis
@@ -29,6 +23,7 @@ rule
 
   path
     : path_members { s(:path, *val[0]) }
+    | path_member
     ;
 
   node_test