Drop case normalization from tags and attributes

We trust the user to use proper capitalization for element names and attributes. Most of the time this doesn't matter, except when htmlbars or another engine wants to generate DOM using the tokens. The odd capitalization rules (like the `viewBox` attribute name, or `linearGradient` tag name) must be permitted to pass through.

The alternative approach, requiring that downstream libraries re-normalize `viewbox` to `viewBox`, seems difficult to maintain practically. Trusting the user to do what is right is simpler.

This PR is required for a downstream PR in htmlbars related to SVG element support (https://github.com/tildeio/htmlbars/pull/41).

/cc @mmun