Use smart case (-S) for ag-dired and friends

This will be consistent with how the regular text ag search works.

(I've had this in my personal fork for years and never figured it wasn't pushed upstream until I moved to this new computer 😊)

ag.el

@@ -539,7 +539,7 @@ See also `find-dired'."
          (buffer-name (if ag-reuse-buffers
                           "*ag dired*"
                         (format "*ag dired pattern:%s dir:%s*" regexp dir)))
-         (cmd (concat ag-executable " --nocolor -g '" regexp "' "
+         (cmd (concat ag-executable " --nocolor -S -g '" regexp "' "
                       (shell-quote-argument dir)
                       " | grep -v '^$' | sed s/\\'/\\\\\\\\\\'/ | xargs -I '{}' "
                       insert-directory-program " "