Don't redefine PDF mime type

This stops the following warning happening while rails initializes

```
warning: already initialized constant PDF
```

I think this is happens because action dispatch sets up the mime type by default https://github.com/rails/rails/blob/e20dd73df42d63b206d221e2258cc6dc7b1e6068/actionpack/lib/action_dispatch/http/mime_types.rb#L31

The fix just stops the constant defined if it is already present