Correct disk free checking command on windows

Author: Emile Joubert

src/rabbit_disk_monitor.erl

@@ -167,7 +167,7 @@ get_disk_free(Dir, {unix, Sun})
 get_disk_free(Dir, {unix, _}) ->
     parse_free_unix(rabbit_misc:os_cmd("/bin/df -kP " ++ Dir));
 get_disk_free(Dir, {win32, _}) ->
-    parse_free_win32(rabbit_misc:os_cmd("dir /-C /W " ++ Dir));
+    parse_free_win32(os:cmd("dir /-C /W \"" ++ Dir ++ [$"]));
 get_disk_free(_, _) ->
     unknown.