* Fix ruby gc_test in ruby 2.4

* Initialize global variables to Qnil.

* 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

--pre was not necessary to install google-protobuf when I tried it today (2017-12-06).

* 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

* Fix ruby segment fault

1) rb_ary_new cannot be called during allocate function. During allocate
fucntion, the containing object hasn't been marked and rb_ary_new may
invoke gc to collect containing object.
2) The global map should be marked before allocating it. Otherwise it
may be garbage collected.

* Add test

* Remove commented code

* Fix grammer error

This makes the frame stack per-parser, and per-thread.  Fixes #3250

This makes the frame stack per-parser, and per-thread.  Fixes #3250

* Fixed a bunch of Ruby conformance errors.

* Fixed some more Ruby conformance errors in JSON.

`.quo` return the most exact devision which fixes accuracy problems for the
timestamp coercion

We have some special code in wrap_memcpy.c to ensure that we use the
2.2.5 version of memcpy, for compatibility with older versions of glibc.
However, we need to make sure we only attempt to do this when we are
actually building with glibc, so that the code can also build
successfully against other libc implementations such as musl.