Apply suggestions from code review

Co-authored-by: Chris Ross <Tratcher@Outlook.com>

Author: dougbu

src/System.Net.Http.Formatting/HttpMessageContent.cs

@@ -361,7 +361,7 @@ private void ValidateStreamForReading(Stream stream)
         {
             // Stream is null case should be an extreme, incredibly unlikely corner case. Every HttpContent from
             // the framework (see dotnet/runtime or .NET Framework reference source) provides a non-null Stream
-            // in the ReadAsStringAsync task's return value. Likely need a poorly-designed derived HttpContent
+            // in the ReadAsStreamAsync task's return value. Likely need a poorly-designed derived HttpContent
             // to hit this. Mostly ignoring the fact this message doesn't make much sense for the case.
             if (stream is null || !stream.CanRead)
             {

test/System.Net.Http.Formatting.Test/HttpMessageContentTests.cs

@@ -50,12 +50,7 @@ private static async Task<string> ReadContentAsync(HttpContent content, bool unB
         {
             if (unBuffered)
             {
-                var stream = new MemoryStream();
-                await content.CopyToAsync(stream);
-                stream.Position = 0L;
-
-                // StreamReader will dispose of the Stream.
-                using var reader = new StreamReader(stream);
+                using var reader = new StreamReader(await content.ReadAsStreamAsync());
 
                 return await reader.ReadToEndAsync();
             }