[SPARK-5790][GraphX]: VertexRDD's won't zip properly for `diff` capability (added tests)

Added tests that maropu [created](https://github.com/maropu/spark/blob/1f64794b2ce33e64f340e383d4e8a60639a7eb4b/graphx/src/test/scala/org/apache/spark/graphx/VertexRDDSuite.scala) for vertices with differing partition counts. Wanted to make sure his work got captured /merged as its not in the master branch and I don't believe there's a PR out already for it.

Author: Brennon York <brennon.york@capitalone.com>

Closes #5023 from brennonyork/SPARK-5790 and squashes the following commits:

83bbd29 [Brennon York] added maropu's tests for vertices with differing partition counts