Dataflow runner harness should always use proper coder URNs

This comes from a Python 3 incompatible bug and mentioned in https://github.com/apache/beam/pull/7521/files#r248993766. The fix decode payload unconditionally in Python 2 or 3 but suggested by robertwb:

"the Dataflow runner harness was fixed to always use proper coder URNs rather than stuffing json-ized dataflow v1b3 cloud proto representations into the payload".