Prevent creating invalid closing tags

fixes #2756

src/renderers/dom/shared/ReactDOMComponent.js

@@ -438,7 +438,7 @@ function isCustomComponent(tagName, props) {
  */
 function ReactDOMComponent(tag) {
   validateDangerousTag(tag);
-  this._tag = tag;
+  this._tag = tag.toLowerCase();
   this._renderedChildren = null;
   this._previousStyle = null;
   this._previousStyleCopy = null;
@@ -533,7 +533,7 @@ ReactDOMComponent.Mixin = {
     if (!tagContent && omittedCloseTags[this._tag]) {
       return tagOpen + '/>';
     }
-    return tagOpen + '>' + tagContent + '</' + this._tag + '>';
+    return tagOpen + '>' + tagContent + '</' + this._currentElement.type + '>';
   },
 
   /**
@@ -550,7 +550,7 @@ ReactDOMComponent.Mixin = {
    * @return {string} Markup of opening tag.
    */
   _createOpenTagMarkupAndPutListeners: function(transaction, props) {
-    var ret = '<' + this._tag;
+    var ret = '<' + this._currentElement.type;
 
     for (var propKey in props) {
       if (!props.hasOwnProperty(propKey)) {

src/renderers/dom/shared/__tests__/ReactDOMComponent-test.js

@@ -452,6 +452,16 @@ describe('ReactDOMComponent', function() {
       };
     });
 
+    it('should not duplicate uppercased selfclosing tags', function() {
+      var Container = React.createClass({
+          render: function() {
+            return React.createElement('BR', null);
+          },
+      });
+      var returnedValue = React.renderToString(<Container/>);
+      expect(returnedValue).not.toContain('</BR>');
+    });
+
     it('should warn against children for void elements', function() {
       spyOn(console, 'error');