Ignore casing when testing for html void elements

Fixes #36

lib/oga/xml/lexer.rb

@@ -347,7 +347,7 @@ 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)
+        if html? and HTML_VOID_ELEMENTS.include?(current_element.downcase)
           add_token(:T_ELEM_END)
           @elements.pop
         end

spec/oga/xml/parser/html_void_elements_spec.rb

@@ -29,6 +29,24 @@ describe Oga::XML::Parser do
     end
   end
 
+  context 'void elements with different casing' do
+    before :all do
+      @node_lc = parse('<br>', html: true).children[0]
+      @node_uc = parse('<BR>', html: true).children[0]
+    end
+
+    example 'parse void elements with different casing' do
+      @node_lc.is_a?(Oga::XML::Element).should == true
+      @node_uc.is_a?(Oga::XML::Element).should == true
+    end
+
+    example 'set the name of the void element to match casing' do
+      @node_lc.name.should == 'br'
+      @node_uc.name.should == 'BR'
+    end
+
+  end
+
   context 'void elements with attributes' do
     before :all do
       @node = parse('<link href="foo">', :html => true).children[0]