pgraph: Filter graph should preserve vertices without an edge

There was a bug in filter graph that caused it to not preserve any
vertices that weren't bound to another with an edge. This fixes the bug
and adds a test too.

pgraph/pgraph.go

@@ -506,9 +506,13 @@ func (g *Graph) FilterGraph(name string, vertices []Vertex) (*Graph, error) {
 		return nil, errwrap.Wrapf(err, "could not run FilterGraph() properly")
 	}
 	for k1, x := range g.adjacency {
+		contains := VertexContains(k1, vertices)
+		if contains {
+			newGraph.AddVertex(k1)
+		}
 		for k2, e := range x {
 			//log.Printf("Filter: %s -> %s # %s", k1.Name, k2.Name, e.Name)
-			if VertexContains(k1, vertices) || VertexContains(k2, vertices) {
+			if contains || VertexContains(k2, vertices) {
 				newGraph.AddEdge(k1, k2, e)
 			}
 		}

pgraph/pgraph_test.go

@@ -191,6 +191,28 @@ func TestFilterGraph1(t *testing.T) {
 	}
 }
 
+func TestFilterGraph2(t *testing.T) {
+	G, _ := NewGraph("g5")
+	v1 := NV("v1")
+	v2 := NV("v2")
+	v3 := NV("v3")
+	v4 := NV("v4")
+	e1 := NE("e1")
+	G.AddEdge(v1, v2, e1)
+	G.AddVertex(v3)
+	G.AddVertex(v4)
+
+	save := []Vertex{v1, v2, v3}
+	out, err := G.FilterGraph("new g5", save)
+	if err != nil {
+		t.Errorf("failed with: %v", err)
+	}
+
+	if c, i := len(save), out.NumVertices(); c != i {
+		t.Errorf("should have %d vertices instead of: %d", c, i)
+	}
+}
+
 func TestDisconnectedGraphs1(t *testing.T) {
 	G, _ := NewGraph("g6")
 	v1 := NV("v1")