Support svg unpaired tags in tag helper

[le0pard]

Summary

Currently tag helper works incorrectly for unpaired svg tags - it always add close pair to its.

Example:

tag.svg { tag.use("href" => "#cool-icon") }

Before:

<svg><use href="#cool-icon"></use></svg>

After:

<svg><use href="#cool-icon"></svg>

[le0pard]

Thanks @rafaelfranca