Fails on lowercase `viewbox` property

[Federico Brigante]

While technically not a full SVG, browsers can still read this, but it fails in image-size

var imageSize = require("image-size")
imageSize(Buffer.from('<svg viewbox="0 0 10 10"></svg>', 'utf8'))

// -> TypeError: Invalid SVG

var imageSize = require("image-size")
imageSize(Buffer.from('<svg viewBox="0 0 10 10"></svg>', 'utf8'))

// -> Object {height: 10, width: 10, type: "svg"}

Tested on https://npm.runkit.com/image-size