**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**
I got this warning: `Warning: Extra attributes from the server: preserveAspectRatio,viewBox` during client side hydratation when my react tree contains mixed case attributes such as these svg ones.

**What is the expected behavior?**

No warnings because there are no difference between server and client attributes.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

**React 16.0.0-rc.2**
This worked on 15.*

This seems to happen because the attribute name is put to lowercase here: https://github.com/facebook/react/blob/6552519a211c3e43e3f6cf5b2237c90ae515d423/src/renderers/dom/fiber/ReactDOMFiberComponent.js#L901
but not in the set here:
https://github.com/facebook/react/blob/6552519a211c3e43e3f6cf5b2237c90ae515d423/src/renderers/dom/fiber/ReactDOMFiberComponent.js#L915
As the set removal is done with the lowercased attribute:
https://github.com/facebook/react/blob/6552519a211c3e43e3f6cf5b2237c90ae515d423/src/renderers/dom/fiber/ReactDOMFiberComponent.js#L1011
it stays in the `extraAttributeNames` set and I got wrongly the previous warning.
} else if (propKey === DANGEROUSLY_SET_INNER_HTML) {
          const rawHtml = nextProp ? nextProp[HTML] || '' : '';
          const serverHTML = domElement.innerHTML;
          const expectedHTML = normalizeHTML(domElement, rawHtml);
          if (expectedHTML !== serverHTML) {
            warnForPropDifference(propKey, serverHTML, expectedHTML);
          }
        } else if (propKey === STYLE) {
          
          extraAttributeNames.delete(propKey);
          const expectedStyle = CSSPropertyOperations.createDangerousStringForStyles(
            nextProp,
          );
          serverValue = domElement.getAttribute('style');
          if (expectedStyle !== serverValue) {
            warnForPropDifference(propKey, serverValue, expectedStyle);
          }
        } else if (isCustomComponentTag) {
          
          extraAttributeNames.delete(propKey.toLowerCase());
          serverValue = DOMPropertyOperations.getValueForAttribute(
            domElement,
            propKey,
            nextProp,
          );
          if (nextProp !== serverValue) {
            warnForPropDifference(propKey, serverValue, nextProp);
          }
        } else if (DOMProperty.shouldSetAttribute(propKey, nextProp)) {
          if ((propertyInfo = DOMProperty.getPropertyInfo(propKey))) {