... that do not change schema

Author: Kan Zhang <kzhang@apache.org>

Closes #448 from kanzhang/SPARK-1460 and squashes the following commits:

111e388 [Kan Zhang] silence MiMa errors in EdgeRDD and VertexRDD
91dc787 [Kan Zhang] Taking into account newly added Ordering param
79ed52a [Kan Zhang] [SPARK-1460] Returning SchemaRDD on Set operations that do not change schema