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 e9980412296b6469f39605f48ea527776d7ac718 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 ```