**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.
'should not blow away user-entered text on successful reconnect to a controlled textarea',
          async () => {
            let changeCount = 0;
            await testUserInteractionBeforeClientRender(
              <ControlledTextArea onChange={() => changeCount++} />,
            );
            expect(changeCount).toBe(0);
          },
        );
        it('should not blow away user-selected value on successful reconnect to an uncontrolled select', () =>
          testUserInteractionBeforeClientRender(
            <select defaultValue="Hello">
              <option key="1" value="Hello">Hello</option>
              <option key="2" value="Goodbye">Goodbye</option>
            </select>,
          ));
        it('should not blow away user-selected value on successful reconnect to an controlled select', async () => {
          let changeCount = 0;
          await testUserInteractionBeforeClientRender(
            <ControlledSelect onChange={() => changeCount++} />,
          );
          expect(changeCount).toBe(0);
        });
      });
    });
  });
  describe('context', function() {
    let PurpleContext, RedContext;
    beforeEach(() => {
      class Parent extends React.Component {