Print email metadata on preview without body (#663)

Why:

* In a setup where one uses a email service with transactional templates
  or any other mechanism where the system does not actually render the
  email content but instead triggers an external service to generate the
  emails, previewing sent emails in a development environment is pretty
  much useless since nothing is shown. Instead developers have to use
  the logs to try and parse the relevant email information.

This change addresses the issue by:

* Including the email information as metadata when the email has no
  body. This shows the data being sent to external service to generate
  the email, thus providing a way for developers to check the relevant
  values in a development environment.

lib/bamboo/plug/sent_email_viewer/index.html.eex

@@ -257,6 +257,11 @@
           <% else %>
             <h3 class="email-detail-body-label">No text body</h3>
           <% end %>
+
+          <%= unless @selected_email.html_body || @selected_email.text_body do %>
+            <h3 class="email-detail-body-label">Metadata</h3>
+            <pre><code><%= inspect(@selected_email, pretty: true) %></pre></code>
+          <% end %>
         </section>
       </section>
     </main>

test/lib/bamboo/plug/sent_email_viewer_plug_test.exs

@@ -282,6 +282,19 @@ defmodule Bamboo.SentEmailViewerPlugTest do
     assert conn.resp_body =~ "Email not found"
   end
 
+  test "shows email metadata when there is no html or text body" do
+    email = normalize_and_push(:email, html_body: nil)
+    selected_email_id = SentEmail.all() |> Enum.at(0) |> SentEmail.get_id()
+    conn = conn(:get, "/sent_emails/foo/#{selected_email_id}")
+
+    conn = AppRouter.call(conn, nil)
+
+    assert conn.status == 200
+    assert {"content-type", "text/html; charset=utf-8"} in conn.resp_headers
+    assert conn.resp_body =~ "Metadata"
+    assert conn.resp_body =~ "#{inspect(email, pretty: true)}"
+  end
+
   defp newest_email do
     List.first(SentEmail.all())
   end