**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.
failedBoundaries = new Set();
      }
      failedBoundaries.add(boundary);
       This method is unsafe outside of the begin and complete phases.
       We might be in the commit phase when an error is captured.
       The risk is that the return path from this Fiber may not be accurate.
       That risk is acceptable given the benefit of providing users more context.
      const componentStack = getStackAddendumByWorkInProgressFiber(failedWork);
      const componentName = getComponentName(failedWork);
       Add to the collection of captured errors. This is stored as a global
       map of errors and their component stack location keyed by the boundaries
       that capture them. We mostly use this Map as a Set; it's a Map only to
       avoid adding a field to Fiber to store the error.
      if (capturedErrors === null) {
        capturedErrors = new Map();
      }
      const capturedError = {
        componentName,
        componentStack,
        error,
        errorBoundary: errorBoundaryFound ? boundary.stateNode : null,
        errorBoundaryFound,
        errorBoundaryName,
        willRetry,
      };
      capturedErrors.set(boundary, capturedError);
      try {
        logCapturedError(capturedError);
      } catch (e) {