PTransform's should always set a Coder on their output PCollections

[kennknowles]

See discussion on https://lists.apache.org/thread.html/1dde0b5a93c2983cbab5f68ce7c74580102f5bb2baaa816585d7eabb@%3Cdev.beam.apache.org%3E

Imported from Jira BEAM-2686. Original Jira may contain additional context.
Reported by: jkff.