make cookies.delete("not_existing") not fail (#1132)

make ```cookies.delete("not_existing")``` have same behaviour as ```session.delete("not_existing")```, when delete key that not exists, return nil instead of fail

src/lucky/cookies/cookie_jar.cr

@@ -49,7 +49,7 @@ class Lucky::CookieJar
   end
 
   def delete(key : Key) : Nil
-    if cookie = cookies[key.to_s]
+    if cookie = cookies[key.to_s]?
       cookie.expires(1.year.ago).value("")
       set_cookies[key.to_s] = cookie
     end