make the "default" filter match the behavior of Jinja

Falsy values shouldn't be replaced by the default value unless the optional "bool" parameter is truthy.  See http://jinja.pocoo.org/docs/templates/#default.

![screen shot 2013-10-27 at 10 50 50 pm](https://f.cloud.github.com/assets/1527302/1417773/a57f6b40-3f9c-11e3-897d-0a9811e3ce10.png)

This is a continuation of the PR here: https://github.com/jlongster/nunjucks/pull/141