In order to determine which nodes are in a template, for most nodes one can query the `nodelist`s recursively.  For `if` tags, this does not work if the tag has an `else`: in that case because of [this](https://github.com/Shopify/liquid/blob/master/lib/liquid/tags/if.rb#L72), the `nodelist` returns only the nodes in the last condition.  This seemed like an inconsistency in the interface.

Apologies again for this making it into this PR.