**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.
{options}
            </select>,
          );
          expectSelectValue(e, ['bar', 'baz']);
        },
      );
      itRenders(
        'a select with a multiple values and readOnly',
        async render => {
          const e = await render(
            <select value={['bar', 'baz']} multiple={true} readOnly={true}>
              {options}
            </select>,
          );
          expectSelectValue(e, ['bar', 'baz']);
        },
      );
      itRenders(
        'a select with a value and no onChange/readOnly',
        async render => {
          
          
          const e = await render(<select value="bar">{options}</select>, 1);
          expectSelectValue(e, 'bar');
        },
      );
      itRenders('a select with a defaultValue', async render => {
        const e = await render(<select defaultValue="bar">{options}</select>);
        expectSelectValue(e, 'bar');
      });