[SPARK-16620][CORE] Add back the tokenization process in `RDD.pipe(command: String)`

## What changes were proposed in this pull request?

Currently `RDD.pipe(command: String)`:
- works only when the command is specified without any options, such as `RDD.pipe("wc")`
- does NOT work when the command is specified with some options, such as `RDD.pipe("wc -l")`

This is a regression from Spark 1.6.

This patch adds back the tokenization process in `RDD.pipe(command: String)` to fix this regression.

## How was this patch tested?
Added a test which:
- would pass in `1.6`
- _[prior to this patch]_ would fail in `master`
- _[after this patch]_ would pass in `master`

Author: Liwei Lin <lwlin7@gmail.com>

Closes #14256 from lw-lin/rdd-pipe.