cc davies sun-rui

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #6620 from shivaram/sparkr-read-schema and squashes the following commits:

16a6726 [Shivaram Venkataraman] Fix loadDF to pass schema Also add a unit test
a229877 [Shivaram Venkataraman] Use wrapper function to DataFrameReader
ee70ba8 [Shivaram Venkataraman] Support user-specified schema in read.df