Use Zlib::SYNC_FLUSH so buffer does not grow until finished

With JRuby implementation deflate would always return empty string
without Zlib::SYNC_FLUSH. That can cause memory problems when large
files are in deflate buffer as whole and red there with finish call at
once.

lib/zip/deflater.rb

@@ -13,7 +13,7 @@ module Zip
       val   = data.to_s
       @crc  = Zlib.crc32(val, @crc)
       @size += val.bytesize
-      buffer = @zlib_deflater.deflate(data)
+      buffer = @zlib_deflater.deflate(data, Zlib::SYNC_FLUSH)
       if buffer.empty?
         @output_stream
       else
@@ -22,7 +22,9 @@ module Zip
     end
 
     def finish
-      @output_stream << @encrypter.encrypt(@zlib_deflater.finish) until @zlib_deflater.finished?
+      buffer = @zlib_deflater.finish
+      @output_stream << @encrypter.encrypt(buffer) unless buffer.empty?
+      @zlib_deflater.close
     end
 
     attr_reader :size, :crc