Improve AddNLog with LoggingConfiguration to handle custom LogFactory-instance (#407)

Author: snakefoot

src/NLog.Extensions.Logging/Extensions/ConfigureExtensions.cs

@@ -118,9 +118,10 @@ public static ILoggingBuilder AddNLog(this ILoggingBuilder builder, LoggingConfi
         {
             AddNLogLoggerProvider(builder.Services, null, options, (serviceProvider, config, options) =>
             {
-                var provider = CreateNLogLoggerProvider(serviceProvider, config, options);
+                var logFactory = configuration?.LogFactory ?? LogManager.LogFactory;
+                var provider = CreateNLogLoggerProvider(serviceProvider, config, options, logFactory);
                 // Delay initialization of targets until we have loaded config-settings
-                LogManager.Configuration = configuration;
+                logFactory.Configuration = configuration;
                 return provider;
             });
             return builder;