Merge c42a8a326c into 2c67b48c21

To cover the below logic in `Graph class`

```javascript
  deleteEdge(edge) {
    // omit

    const start = this.getVertexByKey(edge.startVertex.getKey());
    const end = this.getVertexByKey(edge.endVertex.getKey());

    start.deleteEdge(edge);
    end.deleteEdge(edge);
  }
```

src/data-structures/graph/__test__/Graph.test.js

@@ -246,6 +246,9 @@ describe('Graph', () => {
     expect(graph.getAllEdges().length).toBe(2);
     expect(graph.getAllEdges()[0].getKey()).toBe(edgeBC.getKey());
     expect(graph.getAllEdges()[1].getKey()).toBe(edgeAC.getKey());
+    
+    expect(vertexA.hasEdge(edgeAB)).toBe(false);
+    expect(vertexB.hasEdge(edgeAB)).toBe(false)
   });
 
   it('should should throw an error when trying to delete not existing edge', () => {