Add and apply Style/StringChars rubocop rule

This rule was added in Rubocop 1.12.0.

.rubocop.yml

@@ -538,6 +538,9 @@ Style/SingleLineMethods:
 Style/SoleNestedConditional:
   Enabled: true
 
+Style/StringChars:
+  Enabled: true
+
 Style/StringLiterals:
   EnforcedStyle: double_quotes
 

app/controllers/management/users_controller.rb

@@ -47,7 +47,7 @@ class Management::UsersController < Management::BaseController
     end
 
     def user_without_email
-      new_password = "aAbcdeEfghiJkmnpqrstuUvwxyz23456789$!".split("").sample(10).join
+      new_password = "aAbcdeEfghiJkmnpqrstuUvwxyz23456789$!".chars.sample(10).join
       @user.password = new_password
       @user.password_confirmation = new_password