Ensure SAX after_element receives meaningful args

This changes the behaviour of after_element when parsing documents using the SAX
parsing API. Previously it would always receive a nil argument, which is kinda
pointless. This commit changes that by making sure it receives a namespace name
(if any) and the element name.

This fixes #54.

lib/oga/xml/sax_parser.rb

@@ -37,6 +37,26 @@ module Oga
     # For information on the callback arguments see the documentation of the
     # corresponding methods in {Oga::XML::Parser}.
     #
+    # ## Element Callbacks
+    #
+    # The SAX parser changes the behaviour of both `on_element` and
+    # `after_element`. The latter in the regular parser only takes a
+    # {Oga::XML::Element} instance. In the SAX parser it will instead take a
+    # namespace name and the element name. This eases the process of figuring
+    # out what element a callback is associated with.
+    #
+    # An example:
+    #
+    #     class SaxHandler
+    #       def on_element(namespace, name, attrs = {})
+    #         # ...
+    #       end
+    #
+    #       def after_element(namespace, name)
+    #         puts name # => "foo", "bar", etc
+    #       end
+    #     end
+    #
     class SaxParser < Parser
       ##
       # @param [Object] handler The SAX handler to delegate callbacks to.
@@ -52,12 +72,46 @@ module Oga
       instance_methods.grep(/^(on_|after_)/).each do |method|
         eval <<-EOF, nil, __FILE__, __LINE__ + 1
         def #{method}(*args)
-          @handler.#{method}(*args) if @handler.respond_to?(:#{method})
+          run_callback(:#{method}, *args)
 
           return
         end
         EOF
       end
+
+      ##
+      # Manually overwrite `on_element` so we can ensure that `after_element`
+      # always receives the namespace and name.
+      #
+      # @see [Oga::XML::Parser#on_element]
+      # @return [Array]
+      #
+      def on_element(namespace, name, attrs = {})
+        run_callback(:on_element, namespace, name, attrs)
+
+        return namespace, name
+      end
+
+      ##
+      # Manually overwrite `after_element` so it can take a namespace and name.
+      # This differs a bit from the regular `after_element` which only takes an
+      # {Oga::XML::Element} instance.
+      #
+      # @param [Array] namespace_with_name
+      #
+      def after_element(namespace_with_name)
+        run_callback(:after_element, *namespace_with_name)
+      end
+
+      private
+
+      ##
+      # @param [Symbol] method
+      # @param [Array] args
+      #
+      def run_callback(method, *args)
+        @handler.send(method, *args) if @handler.respond_to?(method)
+      end
     end # SaxParser
   end # XML
 end # Oga