All tests passing on JRuby extension

[headius]

This PR fixes all fixable test failures in the stringio test suite when running on JRuby with the Java-based extension.

[headius]

@nobu Hello! Maybe you can help me figure out this last failure, because it has me stumped.

Failure: test_write_encoding_conversion(TestStringIO):
  
  1. [1/4] Assertion for "\"🍣\""
     | Encoding::CompatibilityError expected but nothing was raised.
  
  2. [2/4] Assertion for "\"あ🍣\""
     | Encoding::CompatibilityError expected but nothing was raised.
  
  3. [4/4] Assertion for "[\"a\", \"🍣\"]"
     | Encoding::CompatibilityError expected but nothing was raised.
  <false> is not true.
/Users/headius/work/jruby/lib/ruby/gems/shared/gems/test-unit-ruby-core-1.0.5/lib/core_assertions.rb:532:in 'assert'
/Users/headius/work/jruby/lib/ruby/gems/shared/gems/test-unit-ruby-core-1.0.5/lib/core_assertions.rb:766:in 'assert_all_assertions'
/Users/headius/work/stringio/test/stringio/test_stringio.rb:229:in 'test_write_encoding_conversion'

I've traced through the code in JRuby and CRuby and the first example here eventually gets to these lines in enc_compatible_latter:

https://github.com/ruby/ruby/blob/4ac75f6f6453bbf3c89f5b9ae02a03085b506ed5/encoding.c#L1086-L1087

At this point, str1 is the blank Windows-31J string inside the StringIO, and str2 is the UTF-8 "🍣" character. The test above returns enc2 because str1 is empty, Windows-31J is not ASCII-compatible, and str2 is not all 7-bit ASCII.

But test_write_encoding_conversion wants this to fail! I can't figure out why or how it fails on CRuby because it seems to follow the path I describe, which will always return UTF-8 and not error.

[headius]

@nobu After teaching myself how to step-debug CRuby again I figured out the problem: my version always passed str1 as the first argument to rb_enc_check. This allowed enc2 to be chosen if str1 was blank during enc_compatible_latter. This is the cause of all three failures.

I have a hack coming to duplicate this behavior in StringIO for JRuby until JRuby can add a version of rb_enc_check that can accept an encoding for the first argument.

[headius]

Oddly enough, the JRuby on macos job failed so I only added JRuby on windows to CI. I have filed jruby/jruby#8642 for the issue.

@nobu @hsbt This can be merged and released any time, let me know if you have any comments.

[headius]

I just discovered some regressions and additional failures I can fix. Please wait to merge.

[headius]

I have pushed the remaining fixes needed to have all tests and all specs green. Everything here and in ruby/spec will be green once jruby/jruby#8643 has been merged to JRuby master and deployed as jruby-head. I tried to move that process along, but it may take another day to settle.

Once everything runs green this can be merged and released.

[headius]

@hsbt @nobu I have merged the fixes in and we can release any time. Let me know if I should do that.

[kou]

I'll release a new version in a few days.

[kou]

Done.

[headius]

@kou Thank you!