Change PROMPT_S of simple-prompt

When input `"` or `/` with simple-prompt,

Before:
`"` or `/`
(prompt disappeared and indent is changed)

After:
`"> "` or `/> /`
(indent is unchanged since `>> `)

lib/irb/init.rb

@@ -83,7 +83,7 @@ module IRB # :nodoc:
       :SIMPLE => {
         :PROMPT_I => ">> ",
         :PROMPT_N => ">> ",
-        :PROMPT_S => nil,
+        :PROMPT_S => "%l> ",
         :PROMPT_C => "?> ",
         :RETURN => "=> %s\n"
       },