It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
};
      testUnknownAttributeAssignment({hello: 'world'}, '[object Object]');
      testUnknownAttributeAssignment(lol, 'lol');
    });
    it('removes symbols and warns', () => {
      spyOn(console, 'error');
      testUnknownAttributeRemoval(Symbol('foo'));
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Invalid prop `unknown` on <div> tag. Either remove this ' +
          'prop from the element, or pass a string or number value to keep it ' +
          'in the DOM. For details, see https://fb.me/react-unknown-prop\n' +
          '    in div (at **)',
      );
      expectDev(console.error.calls.count()).toBe(1);
    });
    it('removes functions and warns', () => {
      spyOn(console, 'error');
      testUnknownAttributeRemoval(function someFunction() {});
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Invalid prop `unknown` on <div> tag. Either remove this ' +
          'prop from the element, or pass a string or number value to ' +
          'keep it in the DOM. For details, see ' +
          'https://fb.me/react-unknown-prop\n' +
          '    in div (at **)',
      );
      expectDev(console.error.calls.count()).toBe(1);
    });
  });
});