[SPARK-17805][PYSPARK] Fix in sqlContext.read.text when pass in list of paths

## What changes were proposed in this pull request?
If given a list of paths, `pyspark.sql.readwriter.text` will attempt to use an undefined variable `paths`.  This change checks if the param `paths` is a basestring and then converts it to a list, so that the same variable `paths` can be used for both cases

## How was this patch tested?
Added unit test for reading list of files

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #15379 from BryanCutler/sql-readtext-paths-SPARK-17805.