Fix .add

[5saviahv]

if other is cheerio object, _make will return it as unchanged. So if add changes something there it also affects original other object.

Should fix #834

[fb55]

Nice one! I am a bit surprised that this wasn't implemented this way before, as prevObject wasn't updated before.

Also, we should rename _make to pushStack, to match jQuery — I've opened #1665 to track.