Add test for domain exception filter for case when custom mapper returns null.

Author: Matthew Thornton

test/Winton.DomainModelling.AspNetCore.Tests/DomainExceptionFilterTests.cs

@@ -38,6 +38,24 @@ public sealed class OnException : DomainExceptionFilterTests
                 }
             };
 
+            [Fact]
+            private void ShouldDefaultToBadRequestResultIfCustomMapperReturnsNull()
+            {
+                var exceptionContext =
+                    new ExceptionContext(
+                        new ActionContext(new DefaultHttpContext(), new RouteData(), new ActionDescriptor()),
+                        new List<IFilterMetadata>())
+                    {
+                        Exception = new DomainException("Foo")
+                    };
+                var filter = new DomainExceptionFilter((exception, response) => null);
+
+                filter.OnException(exceptionContext);
+
+                exceptionContext.Result.Should().BeEquivalentTo(
+                    new BadRequestObjectResult(new ErrorResponse(new DomainException("Foo"))));
+            }
+
             [Fact]
             private void ShouldNotSetResultIfNotATypeOfDomainException()
             {