**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.
return (
              <input
                value={this.state.value}
                onChange={this.handleChange.bind(this)}
              />
            );
          }
        };
        ControlledTextArea = class extends React.Component {
          constructor() {
            super();
            this.state = {value: 'Hello'};
          }
          handleChange(event) {
            if (this.props.onChange) {
              this.props.onChange(event);
            }
            this.setState({value: event.target.value});
          }
          render() {
            return (
              <textarea
                value={this.state.value}
                onChange={this.handleChange.bind(this)}
              />
            );
          }
        };
        ControlledCheckbox = class extends React.Component {
          constructor() {