Use vector-friendly comparison for packages argument.

Otherwise, `sparkR.init()` with multiple `sparkPackages` results in this warning:

```
Warning message:
In if (packages != "") { :
  the condition has length > 1 and only the first element will be used
```

Author: trestletech <jeff.allen@trestletechnology.net>

Closes #7701 from trestletech/compare-packages and squashes the following commits:

72c8b36 [trestletech] Correct function name.
c52db0e [trestletech] Added test for multiple packages.
3aab1a7 [trestletech] Use vector-friendly comparison for packages argument.