Buffer.isBuffer() returns explicit true/false, fixed for non-Uint8Array implementation

After upgrading Browserify to v3 on a project, our tests started failing as `Buffer.isBuffer()` didn't explicitly return `false` if not a buffer.

I also noticed that `Buffer.isBuffer()` would return `false` each time if using the non-`Uint8Array` fallback; this pull request fixes that, too.