Add a default-initarg of nil for DISTINFO-SUBSCRIPTION-URL slot in dists.
Add a default-initarg of nil for DISTINFO-SUBSCRIPTION-URL slot in dists.

CodyReichert/quicklisp-client,quicklisp/quicklisp-client,arnoo/quicklisp-client,daewok/quicklisp-client,PuercoPop/quicklisp-client,tamurashingo/quicklisp-client