Clarified error in read_sas method when buffer object provided withou…
…t format

 - [x] closes #14947
 - [x] tests added / passed
 - [X] passes ``git diff upstream/master | flake8 --diff``
 - [X] whatsnew entry

Added three lines to sasreader.py immediately following line 33 (if format==None:) to handle the case when a buffer object is provided without a format='sas7bdat' or format='xport' situation. Method otherwise works splendidly when a filepath is provided, but a buffer object fails. This is an issue when using sasreader directly on SFTP file objects. I am unaware of any bug request (and am happy to open one), but I came across this issue when using the library.