D-Bus: Fix string-marshalled enum handling in async GDBus servers
D-Bus: Fix string-marshalled enum handling in async GDBus servers

tjol/vala-tjol,kamanashisroy/aroop,frida/vala,severinh/vala,playya/vala,rvu95/vala,Distrotech/vala,frida/vala,severinh/vala,frida/vala,xdave/pvala,kamanashisroy/aroop,flobrosch/vala-fixes,spidermonkdust/vala,jessevdk/vala,GNOME/vala,GNOME/vala,flobrosch/vala-fixes,frida/vala,tjol/vala-tjol,jessevdk/vala,tjol/vala-tjol,frida/vala,GNOME/vala,flobrosch/vala-fixes,spidermonkdust/vala,jessevdk/vala,severinh/vala,GNOME/vala,spidermonkdust/vala,rvu95/vala,Distrotech/vala,rvu95/vala,kamanashisroy/aroop,Distrotech/vala,playya/vala,playya/vala