gzip action filter (mostly for web api)

GzipActionFilter.cs

/// <summary>
    /// Enables caching by setting cache-control to private (instead of no-cache).
    /// By default also enables etags, and sets cache-control max-age to 60 seconds.
    /// </summary>
    public class GzipActionFilter : ActionFilterAttribute
    {
        public override async  Task OnActionExecutedAsync(HttpActionExecutedContext context, CancellationToken cancellationToken)
        {
            if (context.Request.Headers.AcceptEncoding.All(i => i.Value != "gzip")) return;
            if (context.Response.Content == null) return;

            using (var compressedStream = new MemoryStream())
            {
                //the gzip stream compresses whatever's written to it, NOT the stream passed to it.
                //that stream is where it writes the output.
                //That seems super confusing to me :/
                using (var gZipStream = new GZipStream(compressedStream, CompressionMode.Compress))
                {
                    //copies the existing content to the gzip stream
                    var existingContent = await context.Response.Content.ReadAsStreamAsync();
                    existingContent.CopyTo(gZipStream);
                }

                //due to nonsense around trying to access closed streams, we just copy to an array rather than
                //use the stream directly.
                context.Response.Content = new ByteArrayContent(compressedStream.ToArray());
            }

            context.Response.Content.Headers.ContentEncoding.Add("gzip");
        }
    }