Fixes <http://bugs.gnu.org/13176>.
* module/srfi/srfi-37.scm (args-fold)[short-option-argument]: When ARGS
is a pair, always set it to its cdr.
* test-suite/tests/srfi-37.test ("SRFI-37")["short option with optional
argument omitted", "short option with optional argument provided"]:
New tests.