OPTIONS requests return a 500 error

```
% curl -vv -XOPTIONS https://latest.datasette.io/
*   Trying 216.58.195.83:443...
> OPTIONS / HTTP/1.1
> Host: latest.datasette.io
> User-Agent: curl/7.70.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 500 Internal Server Error
```