**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.
if (!Array.isArray(selected)) {
          selected = [selected];
        }
        
        
        expect(element.getAttribute('value')).toBe(null);
        expect(element.getAttribute('defaultValue')).toBe(null);
        ['foo', 'bar', 'baz'].forEach(value => {
          const expectedValue = selected.indexOf(value) !== -1;
          var option = element.querySelector(`#${value}`);
          expect(option.selected).toBe(expectedValue);
        });
      };
      itRenders('a select with a value and an onChange', async render => {
        const e = await render(
          <select value="bar" onChange={() => {}}>{options}</select>,
        );
        expectSelectValue(e, 'bar');
      });
      itRenders('a select with a value and readOnly', async render => {
        const e = await render(
          <select value="bar" readOnly={true}>{options}</select>,
        );
        expectSelectValue(e, 'bar');
      });
      itRenders(
        'a select with a multiple values and an onChange',
        async render => {
          const e = await render(
            <select value={['bar', 'baz']} multiple={true} onChange={() => {}}>