[SPARK-15684][SPARKR] Not mask startsWith and endsWith in R

## What changes were proposed in this pull request?

In R 3.3.0, startsWith and endsWith are added. In this PR, I make the two work in SparkR.
1. Remove signature in generic.R
2. Add setMethod in column.R
3. Add unit tests

## How was this patch tested?
Manually test it through SparkR shell for both column data and string data, which are added into the unit test file.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #13476 from wangmiao1981/start.