dillon1000/react

Warn if upper-case tags are used

In #2756 we ended up using toLowerCase to allow case insensitive HTML tags.
However, this requires extra processing every time we access the tag or
at least we need to process it for mount and store it in an extra field
which wastes memory.

So instead, we can just enforce case sensitivity for HTML since this might
matter for the XML namespaces like SVG anyway.