lazily-initialise surroundings ivar

Hey there. I've got a rails app using Mongoid and Active Model Serializers. When I ```include Surrounded``` to my ```Mongoid::Document```, and AMS tries to figure out whether it wants to serialize it or not, Surrounded explodes because it tries to call ```surroundings.first``` in ```context```. For some reason, the Contextual ```new``` doesn't get called. This may be due to the way Mongoid creates objects, I'm not sure. Either way, if you make the surroundings array lazily initialised you can remove a lot of code. I fully expect there's some subtlety that I'm missing here, but it has stopped my exception from being raised.

What do you think?