title: Strong parameters - manual whitelisting
body: <p>For a new rails app:</p>
  
  <pre><code>rails generate scaffold user name:string
  </code></pre>
  
  <p>This works fine in the controller:</p>
  
  <pre><code>def user_params
      params.require(:user).permit(:name)
  end
  </code></pre>
  
  <p>But this...</p>
  
  <pre><code>def user_params
      params.require(:user).tap do |whitelist|
          whitelist[:name] = params[:user][:name]
      end
  end
  </code></pre>
  
  <p>... gives me an error "ActiveModel::ForbiddenAttributesError"</p>
  
  <p>Why so?  Looking at examples of tap/whitelisting, it should work?</p>