New failures in Rack tests relating to deflater

[headius]

In 9.4.3.0 we fixed an issue in the jzlib-based deflater logic (#7814) but it seems there are still some issues causing failures in the Rack CI:

https://github.com/rack/rack/actions/runs/5747045920/job/15577430896

[bmckeough]

I noticed this recently when looking at the Rack project. All recent commits are failing the jruby test matrix and passing the other ruby versions in the test matrix.

It appears to me that the discrepancy between jruby and the other tested ruby runtimes is that jruby does not infer a GZIP mtime when no last-modified header is present. I'm basing this on the following:

• The Rack::Deflater gzip test that specifies a last-modified header passes when run by jruby.
• The other Rack::Deflater gzip tests which do not specify a last-modified header fail when run by jruby.
• All of the Rack::Deflater gzip tests succeed when run by the other ruby runtimes in the Rack test matrix.
• A review of Rack::Deflater and Rack::Deflater::GzipStream implementations, which populate ::Zlib::GzipWriter's mtime only if headers["last-modified"] is not nil.

Unfortunately it isn't immediately obvious to me where this defaulting logic for GZIP mtime should be added in jruby, if that is in fact the appropriate fix.