Adding guards and more tests (#68)

* Adding single entity by spec method to repository

* Adding guards and more unit tests

src/Infrastructure/Logging/LoggerAdapter.cs

@@ -15,6 +15,7 @@ namespace Infrastructure.Logging
         {
             _logger.LogWarning(message, args);
         }
+
         public void LogInformation(string message, params object[] args)
         {
             _logger.LogInformation(message, args);