title: After gem update: test fail with "Asset was not declared to be precompiled in production"
body: <p>Since I updated several gems all tests fail with the error:</p>
  <blockquote>
  <p>ActionView::Template::Error: Asset was not declared to be precompiled
  in production.</p>
  <p>Add <code>Rails.application.config.assets.precompile += %w( favicons/manifest.json.erb )</code> to <code>config/initializers/assets.rb</code> and restart your server</p>
  <p>app/views/layouts/_faviconsheader.html.erb:14:in <code>_app_views_layouts__faviconsheader_html_erb__1320</code></p>
  <p>app/views/layouts/application.html.erb:21:in <code>_app_views_layouts_application_html_erb__4340</code></p>
  </blockquote>
  <p>The error seems to refer to the partial <code>_faviconsheader.html.erb</code> that includes the line:</p>
  <pre><code>&lt;%= content_tag :link, nil, rel: :manifest, href: image_path(&quot;favicons/manifest.json.erb&quot;) %&gt;
  </code></pre>
  <p>This partial is loaded in <code>application.html.erb</code>: <code>&lt;%= render partial: 'layouts/faviconsheader' %&gt;</code>.</p>
  <p>Any idea what is causing this error and what to do? <strong>Before the gem update all tests passed.</strong></p>
  <p>I use Rails 4.2.5. One of the gems updated was <code>sprockets</code> (updated sprockets to version 3.5.2). I read something on <a href="https://github.com/rails/sprockets-rails/issues/266" rel="noreferrer">github</a> about sprockets 4 having a problem, but I'm not using version 4.</p>
  <hr />
  <p>P.S. Even if I add <code>Rails.application.config.assets.precompile += %w( favicons/manifest.json.erb )</code> to <code>config/initializers/assets.rb</code> the error persists. But even if that would have worked I would have wanted to understand why this problem has come about, without any changes except updating some gems.</p>