implement io.ReaderFrom for gin.ResponseWriter

[aaronjanse]

Follow-up to fix the merge conflict in #1610. Credit to @heyimalex (marked as a co-author in Git) for writing the actual code involved.

This causes io.Copy, which calls io.copyBuffer, to automatically use the sendfile syscall by calling the underlying http.ResponseWriter's ReadFrom implementation [1].

[1] https://github.com/golang/go/blob/7eeec1f6e4/src/io/io.go#L410-L413

This can be a significant performance improvement. For example, downloading a 1 GB file is now 33% faster:

# After this patch
$ hyperfine --warmup 2 "curl 'http://localhost:8080/local/file' -o /dev/null"
Benchmark 1: curl 'http://localhost:8080/local/file' -o /dev/null
  Time (mean ± σ):     338.6 ms ±   7.8 ms    [User: 46.9 ms, System: 288.6 ms]
  Range (min … max):   323.0 ms … 348.4 ms    10 runs

# Before this patch
$ hyperfine --warmup 2 "curl 'http://localhost:8080/local/file' -o /dev/null"
Benchmark 1: curl 'http://localhost:8080/local/file' -o /dev/null
  Time (mean ± σ):     509.9 ms ±  17.3 ms    [User: 82.2 ms, System: 282.3 ms]
  Range (min … max):   489.0 ms … 540.7 ms    10 runs