🐛 Fixed error when using API to search for tags
🐛 Fixed error when using API to search for tags

refs https://github.com/TryGhost/Ghost/commit/a457631a208246fef9aef54b0b60c0a92a4283e1

- `defaultColumnsToFetch` was moved to the CRUD plugin in the referenced
  commit, which makes it a function on `this` instead of `this.prototype`
- this means the function doesn't exist and Admin throws an error when
  you start typing in the search bar because the API 500s
- this commit switches it to `this.defaultColumnsToFetch()`

TryGhost/Ghost,ErisDS/Ghost,ErisDS/Ghost,TryGhost/Ghost,ErisDS/Ghost,janvt/Ghost,janvt/Ghost,TryGhost/Ghost,e10/Ghost,e10/Ghost