Apparently, a component of Rails implements a buffering .write
method which keeps the String buffer around and makes it unsafe
for us to clear it after calling .write.

This caused Rack::Deflater to give empty results when enabled.

Fortunately, per r61631 / a55abcc0,
this misguided optimization was only worth a small (0.5MB) savings
and we still benefit from the majority of the memory savings in
that change.

Thanks to zunda for the bug report.

[ruby-core:90133] [Bug #15356]

Fixes: r61631 (commit a55abcc0)
       ("zlib: reduce garbage on gzip writes (deflate)")

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66268 b2dd03c8-39d4-4d8f-98ff-823fe69b080e