Fix bug in MapNode

MapNode is responsible for PlainMapOperatorBase and not for
MapOperatorBase, but this is a mess right now.

Maybe we should think about cleaning the situation a bit. CollectorMap is the same as FlatMap, only the function in the interface is called map instead of flatMap. I know that this is because of historic reasons but it is still a mess because we handle all three types through the layers. For example, some code in PactCompiler looks like this:

```java
else if (c instanceof MapOperatorBase) {
	n = new CollectorMapNode((MapOperatorBase<?>) c);
}
else if (c instanceof PlainMapOperatorBase) {
	n = new MapNode((PlainMapOperatorBase<?>) c);
}
else if (c instanceof FlatMapOperatorBase) {
	n = new FlatMapNode((FlatMapOperatorBase<?>) c);
}
```