Node incorrect Degree

A Multigraph with a loop should count 2 in the degree, but is counted as if it was 1.

```java
Graph graph = new MultiGraph("graph");
graph.addNode("A");
graph.addEdge("A-A", "A", "A", false);
System.out.println(graph.getNode("A").getDegree());
```
returns 1, not 2.