User-Agent string contains undefined in browserified build

[Zirro]

The default user agent string for jsdom makes use of process.platform which is not supported by browserify's process shim, resulting in this:

Mozilla/5.0 (undefined) AppleWebKit/537.36 (KHTML, like Gecko) jsdom/11.11.0

There's an open issue for it here, but based on the discussion I don't think we can expect a fix to be merged any time soon. Should we provide our own fallback value when process.platform is undefined?

[Zirro]

Another option is to stop advertising process.platform in the user agent altogether (or to replace it with a static value in order to maintain a similarity with other User-Agent strings).

The values returned from Node's process.platform already don't match what browsers put there, and we return the empty string for navigator.platform.

Since custom user-agent strings can be set through the API, it shouldn't be a problem for anyone who needs a specific value.

[domenic]

Keeping this information seems vaguely valuable. I'm thinking maybe we should do a fallback of "unknown OS" in the browser.