Fix issue with single author blog posts

I made a change a few commits ago that let us use multiple authors for a post

```
author:
  - Garrick Aden-Buie
  - Travis Gerke
```

but the current implementation breaks for

```
author: Garrick Aden-Buie
```

Temporary workaround is to use list format

```
author:
  - Garrick Aden-Buie
```