This commit adds two entries to the conformance failure list for Ruby.
It seems that these particular test cases have been failing since they
were introduced in 5140bae3, so there
has not been any regression.

* 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

* Fixed a bunch of Ruby conformance errors.

* Fixed some more Ruby conformance errors in JSON.

About 1.5% of all IEEE754 single-precision numbers require nine
decimal digits to represent accurately.

Add note about JSON tests maybe being wrong.

- Add note about the JSON test maybe not being correct yet.
- Add test to checks the generated names for double underscores to be sure they
  are what is expected.

- String behaviors
- Testing only valid JSON is allowed

Add more JSON tests around underscores

Incase the different runtimes have different generation/implementations for
for the different primitive field types, cover a larger range of the proto
types in the oneof zero tests.

* Add conformance test for zero fields in oneofs.
* Add failures to the "expected" files.

Change-Id: Ief77de7134e05e07b1a7e3970d49880c2d5e6fe9

This involved fixing a few important bugs in the
Ruby implementation -- mostly cases of mixing
upb field types and descriptor types (upb field
types do not distinguish between int/sint/fixed/sfixed
like descriptor types do).

Also added protobuf-specific exceptions so parse
errors can be caught specifically.

Change-Id: Ib49d3db976900b2c6f3455c8b88af52cfb86e036