Shopify/liquid — pull request #152 by boourns (2012-10-22 21:26:44+00:00)
Title: Instantiate blank string once instead of at every comparison

@burke @hornairs @camilo Please review

Running dtrace with the following probe:
```
sudo dtrace -n 'ruby*:::string-create{@[copyinstr(arg1),arg2]=count();}' -p
```

I found that during one normal liquid render, `/lib/liquid/drop.rb:35` would instantiate 3149 Strings.

Making the blank string a class attribute saves us the runtime allocations.