Revert part of the previous commit

- `ReadAsStreamAsync()` introduces unwanted behaviours
  - returned `Stream` cannot be read multiple times and is not recreated on next `ReadAsStreamAsync` call
  - method buffers the `HttpContent`, invalidating the intent of `unBuffered`

Author: dougbu

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

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