Custom collections not processed

I have a site I am working on that uses three custom collections. Even though output:true is set for each, the generated sitemap doesnt contain them.
The issue is the way Liquid handles hashes in for loops. From the [Liquid for Designers docs](https://github.com/Shopify/liquid/wiki/Liquid-for-Designers#for-loops):

> When iterating a hash, `item[0]` contains the key, and `item[1]` contains the value:
> 
> ```liquid
> {% for item in hash %}
>   {{ item[0] }}: {{ item[1] }}
> {% endfor %}
> ```