Fix drupal6-to-jekyll migrations

This addresses issues 2 and 3 as identified in https://github.com/jekyll/jekyll-import/commit/0b33a12ae5b1fa6b793414fbeb4bf44388516177#commitcomment-3631300

Instead of manually serializing the tags/categories list as a string ourselves, we leave it as an array and let to_yaml handle the serialization.

I tested this on my drupal installation with posts that contain tags that contain and don't contain spaces, and with posts that contain and don't contain tags.