[COOK-3776] Cannot use existing user with nginx::source recipe - it always tries to create new system user

https://tickets.opscode.com/browse/COOK-3776

Here is my use-case:
I have an existing user and I run my application server under this user.
I need nginx to run its worker processes under the same user, but nginx::source recipe tries to create new system user with the name specified in node['nginx']['user'] attribute.
It turns out I cannot skip user creation. Here is my patch (first thing that came to my mind). I would like to hear your thoughts about my use case and possible fixes for it.