Downcase attribute names in SSR diff hydration

IE Edge reports some SVG attributes, like stroke and fill, in all
caps. This commit downcases all attribute names so that comparison
when diffing is case in-sensitive.

src/renderers/dom/fiber/ReactDOMFiberComponent.js

@@ -928,8 +928,10 @@ var ReactDOMFiberComponent = {
       var extraAttributeNames: Set<string> = new Set();
       var attributes = domElement.attributes;
       for (var i = 0; i < attributes.length; i++) {
-        // TODO: Do we need to lower case this to get case insensitive matches?
-        var name = attributes[i].name;
+        // Downcase to work around IE Edge, which reports some SVG attributes
+        // in all-caps. See:
+        // https://github.com/facebook/react/pull/10394#issuecomment-320523369
+        var name = attributes[i].name.toLowerCase();
         switch (name) {
           // Built-in attributes are whitelisted
           // TODO: Once these are gone from the server renderer, we don't need