* Reserve unknown in ruby
* Revert ruby tests. Wait for cpp impl for conformance test
* Add conformance test for preserving unknown
* Add unknown field conformance test to csharp failure list.
* Fix comments
* Fix comment
* Fix comments
* Fix typo
* Use stringsink_string directly
* Mark hd unused
* Remove unused encodeunknown_handlerfunc
This better shows the semantic of the API. For already setted fields,
mergeFromString do replacement for singular fields and appending for
repeated fields.