Prevent NullRef on header assert

src/NzbDrone.Integration.Test/Client/ClientBase.cs

@@ -72,7 +72,7 @@ namespace NzbDrone.Integration.Test.Client
         {
             // cache control header gets reordered on net core
             var headers = response.Headers;
-            ((string)headers.Single(c => c.Name == "Cache-Control").Value).Split(',').Select(x => x.Trim())
+            ((string)headers.SingleOrDefault(c => c.Name == "Cache-Control")?.Value ?? string.Empty).Split(',').Select(x => x.Trim())
                 .Should().BeEquivalentTo("no-store, no-cache".Split(',').Select(x => x.Trim()));
             headers.Single(c => c.Name == "Pragma").Value.Should().Be("no-cache");
             headers.Single(c => c.Name == "Expires").Value.Should().Be("-1");