Fix RstReader authors metadata processing

The reader would return a list of authors already, but
METADATA_PROCESSORS['authors'] expects a string.
Added a test case for this (only the HTMLReader had it).
Merge branch 'master' of https://github.com/getpelican/pelican into davidmarble-page-order-by
Addressed comments from @avaris in PR getpelican/pelican#1348