Add test for warning for camelCased unknown props

**what is the change?:**
When we render unknown props, they get converted to lower-case.

This may be unexpected for people, or break what they are expecting to
happen. Let's warn in this case and ask them to explicitly pass the
lower-cased attribute name.

**why make this change?:**
To avoid corner case buggy results for users.

**test plan:**
NOTE: ~~~It does NOT pass right now. This is a known issue.
Should we change and just expect a warning, but allow the attribute
value to be set?

`yarn run test src/renderers/dom/shared/__tests__/ReactDOMAttribute-test.js`

**issue:** facebook#10399

src/renderers/dom/shared/__tests__/ReactDOMAttribute-test.js

@@ -133,5 +133,20 @@ describe('ReactDOM unknown attribute', () => {
       );
       expectDev(console.error.calls.count()).toBe(1);
     });
+
+    it('removes camelCased unknown props and warns', () => {
+      spyOn(console, 'error');
+
+      var el = document.createElement('div');
+      ReactDOM.render(<div hELLo="something" />, el);
+      expect(el.firstChild.hasAttribute('hELLo')).toBe(false);
+      expect(el.firstChild.hasAttribute('hello')).toBe(false);
+
+      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
+        'Warning: Invalid prop `hELLo` on <div> tag. Did you mean `hello`?' +
+          '    in div (at **)',
+      );
+      expectDev(console.error.calls.count()).toBe(1);
+    });
   });
 });