Created method as.data.frame as a synonym for collect().

Author: Oscar D. Lara Yejas <olarayej@mail.usf.edu>
Author: olarayej <oscar.lara.yejas@us.ibm.com>
Author: Oscar D. Lara Yejas <oscar.lara.yejas@us.ibm.com>

Closes #8908 from olarayej/SPARK-10807.