It should be possible to pass a list value to a custom URL converter when building. However, after some other changes to the code to support MultiDict, a regression was introduced that made that difficult.

This changes the behavior so flattening single-item lists only happens when passing MultiDict, where values will always be lists, and are mainly coming from request.args where this behavior makes sense. For plain dicts, neither of these things are done, it is assumed that the user will intentionally pass a list or single item depending on what's appropriate.

Additionally, build no longer filters None items from list values. Either the list will be passed to a URL converter, where it's the converter's job to handle the value passed to it, or it's a query arg, and url_encode already filters out None.

fixes #2249
continues #2261