Fix return type for guard methods and typemapper methods

src/de/steamwar/command/AbstractSWCommand.java

@@ -180,7 +180,7 @@ public abstract class AbstractSWCommand<T> {
             commandSystemWarning(() -> "The method '" + method.toString() + "' is lacking the first parameter of type '" + clazz.getTypeName() + "'");
             return;
         }
-        if (returnType != null && method.getReturnType() != returnType) {
+        if (returnType != null && !method.getReturnType().isAssignableFrom(returnType)) {
             commandSystemWarning(() -> "The method '" + method.toString() + "' is lacking the desired return type '" + returnType.getTypeName() + "'");
             return;
         }