`covary` etc. should reset coder

[nevillelyh]

Right now they only cast underlying SCollection, which might not work for mixing SCollections of ADTs. https://github.com/spotify/scio/blob/main/scio-core/src/main/scala/com/spotify/scio/values/SCollection.scala#L294

https://spotify-foss.slack.com/archives/C015189TFDH/p1620769273059800