Fix case where a variable name is falsy

https://github.com/Shopify/liquid/pull/443 changed the behaviour of `parse` to build Ruby objects right away. This means that the variable `false` would have `@name = false` rather than `@name = 'false'`.

@dylanahsmith @fw42 
I don't think so. checking ```== nil``` in conditionals has nothing to do with strings, does it? Liquid is smarter than that. (see expression.rb)