Use Symbol#equal? instead of Symbol#==

At least on JRuby and Rubinius this can be quite a bit faster. On MRI
the difference is not really significant.

lib/oga/css/parser.rll

@@ -668,7 +668,7 @@ even
   # @return [TrueClass|FalseClass]
   #
   def int_node?(node)
-    return node.type == :int
+    return node.type.equal?(:int)
   end
 
   ##

lib/oga/xml/document.rb

@@ -83,7 +83,7 @@ module Oga
       # @return [TrueClass|FalseClass]
       #
       def html?
-        return type == :html
+        return type.equal?(:html)
       end
 
       ##

lib/oga/xpath/evaluator.rb

@@ -1640,7 +1640,7 @@ module Oga
       def node_matches?(xml_node, ast_node)
         ns, name = *ast_node.children
 
-        if ast_node.type == :type_test
+        if ast_node.type.equal?(:type_test)
           return type_matches?(xml_node, ast_node)
         end
 

lib/oga/xpath/parser.rll

@@ -134,7 +134,7 @@ path_step
       args = val[1][1]
       pred = val[1][2]
 
-      if type == :test
+      if type.equal?(:test)
         # Whenever a bare test is used (e.g. just "A") this actually means
         # "child::A". Handling this on parser level is the easiest.
         if args