`<meta>.content` was a getter-only stub. `next/head` and `next/document`
iterate <meta> tags and assign `.content` during hydration, which threw
`Cannot set property content`, React caught and retried in a tight loop,
the page never mounted, and the error spammed the console thousands of
times. Traveloka and other Next.js sites doing meta management were unusable.
Now reflects the content attribute with both get and set, per spec.
Template's `.content` (DocumentFragment) is unchanged.
Closes#210.
SGavrl
3c3798b197