IIS提供对Chrome Postman的压缩(gzip)响应，但不提供对.NET HttpClient的响应

我已经在运行IIS 7.5的Windows Server 2008 R2机器上使用Web API 2.2创建了REST Web服务.我遇到的问题是，当我通过Google Chrome Postman应用程序发出请求时，Web服务正在返回压缩的响应(Content-Encoding:gzip).但是，当我使用.NET 4.5.1 HttpClient发出相同的请求时，服务器不会返回压缩的响应(Content-Encoding标头为空).这是我的C#代码:

I have created a REST web service using Web API 2.2 on a Windows Server 2008 R2 box running IIS 7.5. The problem that I'm having is that the web service is returning a compressed response (Content-Encoding: gzip) when I make the request through the Google Chrome Postman application. But when I make the same request using the .NET 4.5.1 HttpClient, the server does not return a compressed response (the Content-Encoding header is blank). Here is my C# code:

var handler = new HttpClientHandler();
handler.UseProxy = false;
handler.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
handler.Credentials = CredentialCache.DefaultNetworkCredentials;

var client = new HttpClient(handler);
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
client.DefaultRequestHeaders.AcceptCharset.Add(new StringWithQualityHeaderValue("utf-8"));
client.DefaultRequestHeaders.AcceptLanguage.Add(new StringWithQualityHeaderValue("en-US"));
client.DefaultRequestHeaders.CacheControl = new CacheControlHeaderValue { NoCache = true };
client.DefaultRequestHeaders.Connection.Add("keep-alive");

var response = await client.GetAsync("https://localhost/mywebsite");

注意:我正在使用SSL连接.我可以确认Web API Web服务正在从Postman应用程序请求和HttpClient请求中接收到Accept-Encoding: gzip标头.实际上，两者的请求标头完全相同，只是Connection: keep-alive标头似乎已从HttpClient请求中剥离了.有谁知道为什么该Web服务不能提供对HttpClient的压缩响应?

Note: I'm using an SSL connection. I can confirm that the Web API web service is receiving the Accept-Encoding: gzip header from both the Postman application request and the HttpClient request. In fact, the request headers are exactly the same for both, except that the Connection: keep-alive header seems to be stripped from the HttpClient request. Does anyone have any idea why the web service won't serve a compressed response to the HttpClient?