[SPARK-6121][SQL][MLLIB] simpleString for UDT

`df.dtypes` shows `null` for UDTs. This PR uses `udt` by default and `VectorUDT` overwrites it with `vector`.

jkbradley davies

Author: Xiangrui Meng <meng@databricks.com>

Closes #4858 from mengxr/SPARK-6121 and squashes the following commits:

34f0a77 [Xiangrui Meng] simpleString for UDT