Compression for buf_file plugin not used on restore (master)

[berlincount]

Describe the bug

While buffering a chunk on disk it can be compressed using gzip, but when restoring it, is isn't uncompressed, leading to a broken chunk that's possibly send further on.

To Reproduce

  <buffer>
    @type file
    path /var/log/shopify-fluentd/fluentd/example.buffer
    chunk_limit_size 10M
    total_limit_size 1280M
    flush_interval 5s
    retry_max_interval 30
    retry_forever true
    flush_thread_count 4
    overflow_action block
    compress gzip
  </buffer>

Cause a chunk to be buffered, restart fluentd. The latter should cause the chunk to be unbuffered and uncompressed, but it's loaded in compressed form.

Expected behavior

Chunks should be buffered and unbuffered correctly regardless of compression setting.