[SPARK-17339][SPARKR][CORE] Fix some R tests and use Path.toUri in...

[SPARK-17339][SPARKR][CORE] Fix some R tests and use Path.toUri in SparkContext for Windows paths in SparkR

## What changes were proposed in this pull request?

This PR fixes the Windows path issues in several APIs. Please refer https://issues.apache.org/jira/browse/SPARK-17339 for more details.

## How was this patch tested?

Tests via AppVeyor CI - https://ci.appveyor.com/project/HyukjinKwon/spark/build/82-SPARK-17339-fix-r

Also, manually,

![2016-09-06 3 14 38](https://cloud.githubusercontent.com/assets/6477701/18263406/b93a98be-7444-11e6-9521-b28ee65a4771.png)

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #14960 from HyukjinKwon/SPARK-17339.