Default frontmatter defauls for permalinks not working properly

I have the need for special permalinks for posts only. I attempted this by adding the following to my _config.yml:


```yaml
defaults:
-
  scope:
    path: ""
      type: "post"
    values:
      permalink: :categories/:title
```

If I have a post with a title of "testpost" and a category defined as "blog", I had hoped this setting would cause this file to be generated at `blog/testpost/index.html`. However, Jekyll is actually generating a folder called ":category/:title/" with a folder called "index.html" inside it, which isn't close to what I would expect.

I'd expect permalinks defined this way (and with template variables) to behave just like permalinks, with template variables, that are included in frontmatter at the top of a post.