## 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.