`request.post_vars()` cannot handle multiple values for a key

[simonw]

https://github.com/simonw/datasette/blob/832f76ce26ffb2f3e27a006ff90254374bd90e61/datasette/utils/asgi.py#L138-L140

Datasette GET requests often use ?_facet=category&_facet=size - but right now there's no neat way to handle that with POST data, since this code reduces any multiple values to just one.

This came up while working with <select multiple> (enhanced using Choices.js) in:

• https://github.com/datasette/datasette-acl/issues/24