moving apidiff out of preview (#3595)

release-notes/3.0/api-diff/.Net/3.0.0_System.Net.Http.md

@@ -0,0 +1,20 @@
+# System.Net.Http
+
+``` diff
+ namespace System.Net.Http {
+     public class HttpClient : HttpMessageInvoker {
++        public static IWebProxy DefaultProxy { get; set; }
++        public Version DefaultRequestVersion { get; set; }
+     }
+     public class HttpResponseMessage : IDisposable {
++        public HttpResponseHeaders TrailingHeaders { get; }
+     }
+     public class MultipartContent : HttpContent, IEnumerable, IEnumerable<HttpContent> {
++        protected override Task<Stream> CreateContentReadStreamAsync();
+     }
+     public sealed class ReadOnlyMemoryContent : HttpContent {
++        protected override Task<Stream> CreateContentReadStreamAsync();
+     }
+ }
+```
+