The "fan" didn't urinate in the keeper's bottle, he urinated an empty identical one then went onto the pitch and swapped it. There's a video on Twitter of his doing it.
I completely sympathise with the keeper, and can't honestly say that I wouldn't react in exactly the same way. Also feel for the referee as he is in an awful position there. The problem he has is he won't just have sprayed it at who switched the bottle, assuming he even knew who that was, I would guess he just sprayed it into the crowd. You'd either have to claim an Arsene Wenger and say you didn't see it or send him off, really don't see any other option.
The idiot doing this got someone to film him and post it on social media, so the police should easily be able to identify him, and that can only be classed as an assault. This is the problem with the wise of social media, people being dared, or daring themselves, to do ridiculous things so that they can trend and become "famous". They need to be made to learn that actions have consequences.