Safe async disposal.

Author: JimBobSquarePants

src/ImageSharp.Web/Middleware/ImageSharpMiddleware.cs

@@ -316,11 +316,33 @@ private async Task ProcessRequestAsync(HttpContext context, bool processRequest,
                 }
                 finally
                 {
-                    outStream?.Dispose();
+                    await this.StreamDisposeAsync(outStream);
                 }
             }
         }
 
+        private ValueTask StreamDisposeAsync(Stream stream)
+        {
+            if (stream is null)
+            {
+                return default;
+            }
+
+#if NETCOREAPP2_1
+            try
+            {
+                stream.Dispose();
+                return default;
+            }
+            catch (Exception ex)
+            {
+                return new ValueTask(Task.FromException(ex));
+            }
+#else
+            return stream.DisposeAsync();
+#endif
+        }
+
         private async Task SendResponseAsync(ImageContext imageContext, string key, Stream stream, ImageCacheMetadata metadata)
         {
             imageContext.ComprehendRequestHeaders(metadata.CacheLastWriteTimeUtc, stream.Length);