Fix import of pages with attribute :is_published=false 

When importing a site from fixtures, all pages end up with :is_published set to true, regardless of what is in the fixtures directory. 

This is due to .present? mishandling 'false', treating it [the same way it handles nil and empty strings](http://api.rubyonrails.org/classes/Object.html#method-i-blank-3F). 

Instead, we can use attributes[:is_published].nil? to catch values that are not present, allowing 'true' and 'false' to pass through correctly.