Fix "can't convert Symbol into Integer" occurred in Redmine 3.1

Author: onozaty

app/views/settings/_view_customize_settings.html.erb

@@ -1,4 +1,5 @@
 <p>
   <label for="settings_create_api_access_key"><%= l(:option_create_api_access_key) %></label>
-  <%= check_box_tag 'settings[create_api_access_key]', 1, @settings[:create_api_access_key] %>
+  <%= hidden_field_tag 'settings[create_api_access_key]', '0' %>
+  <%= check_box_tag 'settings[create_api_access_key]', '1', @settings[:create_api_access_key] == '1' %>
 </p>