"or" statement does not work as expected in "if" tests

Testing for the empty value of two Django variables fails when using the HAML syntax but works with the Django templating syntax.

This failed:

``` html
-if myobject.attr1 or myobject.attr2
    %div foobar
```

This worked:

``` html
{% if myobject.attr1 or myobject.attr2 %}
    %div foobar
{% endif %}
```

Using hamlpy==0.82.2 and Django==1.6.1.
