added merge_nodes test

skimage/graph/tests/test_rag.py

@@ -3,6 +3,7 @@ from skimage import graph
 import random
 
 
+
 def test_rag_merge():
     g = graph.rag.RAG()
     for i in range(10):
@@ -14,6 +15,8 @@ def test_rag_merge():
         y = random.choice(g.neighbors(x))
         g.merge_nodes(x, y)
 
+    np.testing.assert_raises(ValueError,g.merge_nodes,7,9)
+
     idx = g.nodes()[0]
     sorted(g.node[idx]['labels']) == range(10)
     assert g.edges() == []