* Add discard unknown API in ruby.
* Add test for oneof message field.
* Add TestUnknown to represent unknown field data clearly.
* Only serialize the message with unknown fields itself in test.
* Move discard_unknown from Message to Google.Protobuf
- A golden-file test that ensures protoc produces known-valid output.
- A Ruby test that loads that golden file and ensures it actually works
with the extension.
This split strategy allows us to test end-to-end without needing to
integrate the Ruby gem build system and the protoc build system. This is
desirable because we do not want a gem build/install to depend on
building protoc, and we do not want building protoc to depend on
building and testing the gem.
