Fix DecoderOptions handler.

Author: JimBobSquarePants

src/ImageSharp.Web/Middleware/ImageSharpMiddleware.cs

@@ -343,10 +343,9 @@ private async Task ProcessRequestAsync(
 
                         using (Stream inStream = await sourceImageResolver.OpenReadAsync())
                         {
-                            DecoderOptions decoderOptions = new() { Configuration = this.options.Configuration };
-
                             // TODO: Do we need some way to set options based upon processors?
-                            await this.options.OnBeforeLoadAsync.Invoke(imageCommandContext, decoderOptions);
+                            DecoderOptions decoderOptions = await this.options.OnBeforeLoadAsync.Invoke(imageCommandContext, this.options.Configuration)
+                                ?? new() { Configuration = this.options.Configuration };
 
                             FormattedImage? image = null;
                             try

src/ImageSharp.Web/Middleware/ImageSharpMiddlewareOptions.cs

@@ -27,7 +27,7 @@ public class ImageSharpMiddlewareOptions
     };
 
     private Func<ImageCommandContext, Task> onParseCommandsAsync = _ => Task.CompletedTask;
-    private Func<ImageCommandContext, DecoderOptions, Task> onBeforeLoadAsync = (_, _) => Task.CompletedTask;
+    private Func<ImageCommandContext, Configuration, Task<DecoderOptions?>> onBeforeLoadAsync = (_, _) => Task.FromResult<DecoderOptions?>(null);
     private Func<FormattedImage, Task> onBeforeSaveAsync = _ => Task.CompletedTask;
     private Func<ImageProcessingContext, Task> onProcessedAsync = _ => Task.CompletedTask;
     private Func<HttpContext, Task> onPrepareResponseAsync = _ => Task.CompletedTask;
@@ -118,7 +118,7 @@ public Func<ImageCommandContext, Task> OnParseCommandsAsync
     /// Gets or sets the method that can be used to used to augment decoder options.
     /// This is called before the image is decoded and loaded for processing.
     /// </summary>
-    public Func<ImageCommandContext, DecoderOptions, Task> OnBeforeLoadAsync
+    public Func<ImageCommandContext, Configuration, Task<DecoderOptions?>> OnBeforeLoadAsync
     {
         get => this.onBeforeLoadAsync;