fix stylesheet cache to recover if file is on disk

spec/controllers/stylesheets_controller_spec.rb

@@ -0,0 +1,25 @@
+require 'spec_helper'
+
+describe StylesheetsController do
+
+  it 'can survive cache miss' do
+    DiscourseStylesheets.cache.clear
+    DiscourseStylesheets.stylesheet_link_tag('desktop_rtl')
+
+    StylesheetCache.destroy_all
+
+    # digestless
+    get :show, name: 'desktop_rtl'
+    expect(response).to be_success
+
+    # tmp folder destruction and cached
+    `rm #{DiscourseStylesheets.cache_fullpath}/*`
+
+    get :show, name: 'desktop_rtl'
+    expect(response).to be_success
+
+    # there is an edge case which is ... disk and db cache is nuked, very unlikely to happen
+
+  end
+
+end