[SPARK-2062][GraphX] VertexRDD.apply does not use the mergeFunc

create verticesDeduplicate with reduceByKey, using mergeFunc
then proceed with verticesDedup

But this is not tested and I want to add a test on VertexRDD.apply, 
because it need Edges, should I place it in VertexRDDSuite or else?
Looks like this line is too long - it would be great if you could wrap it. Also, I think the Spark style is for parameter lists to be indented 4 spaces instead of 2.