Improved HTML void element detection performance.

This ensures we only call String#downcase if we can't find an all lowercased
*and* all uppercased version of the element name. This in turn can save as many
object allocations as there are HTML opening tags.

This fixes #52.

lib/oga/xml/html_void_elements.rb

@@ -24,5 +24,7 @@ module Oga
       'track',
       'wbr'
     ])
+
+    HTML_VOID_ELEMENTS.merge(HTML_VOID_ELEMENTS.map { |name| name.upcase })
   end # XML
 end # Oga

lib/oga/xml/lexer.rb

@@ -322,7 +322,12 @@ module Oga
       # Called on the closing `>` of the open tag of an element.
       #
       def on_element_open_end
-        if html? and HTML_VOID_ELEMENTS.include?(current_element.downcase)
+        return unless html?
+
+        # Only downcase the name if we can't find an all lower/upper version of
+        # the element name. This can save us a *lot* of String allocations.
+        if HTML_VOID_ELEMENTS.include?(current_element) \
+        or HTML_VOID_ELEMENTS.include?(current_element.downcase)
           add_token(:T_ELEM_END)
           @elements.pop
         end