Support props for <meta> elements.

`content`, `httpEquiv`, `charSet` are all needed. We're currently
working around this in `react-page`.

`content` is the risky one here since we previously supporting using
`content` to set the text content. We removed support for that in
e998041 so the risk is minimal, there
just might be some lingering old code.

Fixes #292

Test Plan: Visit `data:text/html,<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" charset="utf-8"></head>`

```
var m = document.querySelector('meta');
m.httpEquiv; // Content-Type
m.httpEquiv = 'foo';
m.httpEquiv; // foo

m.charset; // undefined
m.charSet; // undefined
m.getAttribute('charset'); // utf-8
m.setAttribute('charset', 'bar');
m.getAttribute('charset'); // bar

m.content; // text/html; charset=utf-8
m.content = 'baz';
m.content; // baz
```