<% flash.each do |key, value| %>
  <div class="alert alert-<%= flash_class(key) %> alert-dismissable">
    <button type="button" class="close" data-dismiss="alert" aria-hidden="true">×</button>
    <%= sanitize(value) %>
    <%= flash_icon(key) %>
  </div>
<% end %>

<% flash.each do |key, value| %>
  <div class="alert alert-<%= flash_class(key) %> alert-dismissable">
    <button type="button" class="close" data-dismiss="alert" aria-hidden="true">×</button>
    <%= sanitize(value.to_s) %>
    <%= flash_icon(key) %>
  </div>
<% end %>

Fix error during failsafe response
Fix error during failsafe response

Reason: flashes with booleans (e.g., true/false) fail to be sanitize.
Workaround: convert `value.to_s` before `sanitize`

Question: How did boolean values end up as flash error keys?

```none
F, [2019-01-07T15:38:52.943629 #32257] FATAL -- :
F, [2019-01-07T15:38:52.943701 #32257] FATAL -- : ActionView::Template::Error (undefined method `empty?' for true:TrueClass):
F, [2019-01-07T15:38:52.943879 #32257] FATAL -- :     1: <% flash.each do |key, value| %>
    2:   <div class="alert alert-<%= flash_class(key) %> alert-dismissable">
    3:     <button type="button" class="close" data-dismiss="alert" aria-hidden="true">×</button>
    4:     <%= sanitize(value) %>
    5:     <%= flash_icon(key) %>
    6:   </div>
    7: <% end %>
F, [2019-01-07T15:38:52.943925 #32257] FATAL -- :
F, [2019-01-07T15:38:52.943974 #32257] FATAL -- : app/views/layouts/_bootstrap_flash.html.erb:4:in `block in _app_views_layouts__bootstrap_flash_html_erb___2176619363971200105_47023505854240'
app/views/layouts/_bootstrap_flash.html.erb:1:in `_app_views_layouts__bootstrap_flash_html_erb___2176619363971200105_47023505854240'
app/views/layouts/devise.html.erb:10:in `_app_views_layouts_devise_html_erb__43715729503683185_47023505706140'
I, [2019-01-07T15:38:52.944494 #32257]  INFO -- : Processing by ErrorsController#show as HTML
I, [2019-01-07T15:38:52.944559 #32257]  INFO -- :   Parameters: {"code"=>"500"}
I, [2019-01-07T15:38:52.945207 #32257]  INFO -- :   Rendering errors/500.html.erb within layouts/errors
I, [2019-01-07T15:38:52.945430 #32257]  INFO -- :   Rendered errors/500.html.erb within layouts/errors (0.1ms)
I, [2019-01-07T15:38:52.946142 #32257]  INFO -- :   Rendered layouts/_head.html.erb (0.5ms)
I, [2019-01-07T15:38:52.946505 #32257]  INFO -- :   Rendered layouts/_top_header.html.erb (0.2ms)
I, [2019-01-07T15:38:52.947507 #32257]  INFO -- :   Rendered layouts/_sidebar.html.erb (0.8ms)
I, [2019-01-07T15:38:52.951183 #32257]  INFO -- :   Rendered layouts/_bootstrap_flash.html.erb (3.5ms)
I, [2019-01-07T15:38:52.951289 #32257]  INFO -- :   Rendered layouts/_main_content.html.erb (4.6ms)
I, [2019-01-07T15:38:52.951473 #32257]  INFO -- : Completed 500 Internal Server Error in 7ms
Error during failsafe response: undefined method `empty?' for true:TrueClass
```
