title: SendGrid or Devise Conflict with Mail Gem
body: <p>I just configured ActionMailer to use SendGrid so that I could use it with Devise emails. The email part is working perfectly, but now I can't complete a migration. The error I get is '<strong>TypeError: Mail is not a module</strong>' and is shown in the code block below.</p>
  
  <pre><code>$ rake db:migrate
  rake aborted!
  TypeError: Mail is not a module
  .../ruby-2.3.4/gems/mail-2.7.0/lib/mail.rb:3:in `&lt;top (required)&gt;'
  .../ruby-2.3.4/gems/activesupport-5.0.6/lib/active_support/dependencies.rb:293:in `require'
  .../ruby-2.3.4/gems/activesupport-5.0.6/lib/active_support/dependencies.rb:293:in `block in require'
  .../ruby-2.3.4/gems/activesupport-5.0.6/lib/active_support/dependencies.rb:259:in `load_dependency'
  .../ruby-2.3.4/gems/activesupport-5.0.6/lib/active_support/dependencies.rb:293:in `require'
  </code></pre>
  
  <p>I <em>think</em> the issue has to do with the fact that the ActionMailer gem depends on the Mail gem (which includes a 'mail' module called on mail.rb:3), and there are classes called 'mail' in 'devise/test/mailers/email_changed_test.rb' and 'devise/test/mailers/confirmation_instructions_test.rb' as well.</p>
  
  <p>Maybe this conflict is caused by a rails/gem version incompatibility? (I'm using rails 5.0.4, mail 2.7.0, devise 4.3.0.) Or maybe I'm completely missing the point?</p>