Ruby 3.0 support

[Matthijsy]

I see that there hasn't been a lot of activity here, but we still use this gem to provide WebDav sync for our users. Now we would like to migrate to Ruby 3.0, however there are 2 function calls that were deprecated and are removed in ruby 3.0. It is the URI.escape and URI.unescape method. (https://github.com/chrisroberts/dav4rack/blob/master/lib/dav4rack/controller.rb#L35 and https://github.com/chrisroberts/dav4rack/blob/master/lib/dav4rack/controller.rb#L45).

I am fine with opening an PR to fix this, would you be willing to update them gem in that case? What are those two lines supposed to do? Can they be replaced with URI.encode_www_form or is it encoding an full URI (so including the domain and path). I can open an PR but need to understand it a bit more.

Would be great to hear something and see if we can make it ruby 3.0 compatible