Documented defaults seem not to work

Hello,
I configured the defaults according to the docs in my _config.yml as:
```
defaults:
  - scope:
      path: ""
    values:
      image: /assets/images/image.png
```

However, it does not seem to set the tags accordingly. The `og:image` tag is just missing. If I set up
```
image:
    path: /assets/images/image.png
```
on the respective page however it works.