Use more compatible gzip write api instead of "<<".

Fixes undefined method `<<' for Zlib::GzipWriter on JRuby

lib/rack/deflater.rb

@@ -60,7 +60,7 @@ def each(&block)
         @writer = block
         gzip  =::Zlib::GzipWriter.new(self)
         gzip.mtime = @mtime
-        @body.each { |part| gzip << part }
+        @body.each { |part| gzip.write(part) }
         @body.close if @body.respond_to?(:close)
         gzip.close
         @writer = nil