This patch fixes [[SPARK-9180]](https://issues.apache.org/jira/browse/SPARK-9180).
Users can now set the app name of spark-shell using `spark-shell --name "whatever"`.

Author: Kenichi Maehashi <webmaster@kenichimaehashi.com>

Closes #7512 from kmaehashi/fix-spark-shell-app-name and squashes the following commits:

e24991a [Kenichi Maehashi] use setIfMissing instead of setAppName
18aa4ad [Kenichi Maehashi] fix spark-shell to accept --name option