railsチュートリアル_メイラーのエンコードがquoted-printableになる

前提・実現したいこと

railsチュートリアルの第11章
アカウント有効化するために、メイラーを使用したのですが、エンコードがquoted-printableになり、プレビューに「=3D」などのコードが入ってしまう。
チュートリアルの見本は「7bit」になっている。

メイラーのプレビュー

UserMailer#account_activation: processed outbound mail in 39.1ms
Sent mail to rei@mail.com (8.1ms)
Date: Thu, 11 Apr 2019 14:20:13 +0900
From: noreply@example.com
To: rei@mail.com
Message-ID: <5caece8d36edc_22e63fd7b3711720293fc@xxx.local.mail>
Subject: Account activation
Mime-Version: 1.0
Content-Type: multipart/alternative;
 boundary="--==_mimepart_5caece8d356db_22e63fd7b3711720292f1";
 charset=UTF-8
Content-Transfer-Encoding: 7bit


----==_mimepart_5caece8d356db_22e63fd7b3711720292f1
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: quoted-printable

Hi =E7=B6=BE=E6=B3=A2=E3=83=AC=E3=82=A4,=0D
=0D
Welcome to the Sample App! Click on the link below to activate your accou=
nt:=0D
=0D
http://localhost:3000/account_activations/UIYKxqawmwt9Nx1okrfigg/edit?ema=
il=3Drei%40mail.com=0D
=0D

----==_mimepart_5caece8d356db_22e63fd7b3711720292f1
Content-Type: text/html;
 charset=UTF-8
Content-Transfer-Encoding: quoted-printable

<!DOCTYPE html>=0D
<html>=0D
  <head>=0D
    <meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Dutf=
-8" />=0D
    <style>=0D
      /* Email styles need to be inline */=0D
    </style>=0D
  </head>=0D
=0D
  <body>=0D
    <h1>Sample App</h1>=0D
=0D
<p>Hi =E7=B6=BE=E6=B3=A2=E3=83=AC=E3=82=A4,</p>=0D
=0D
<p>=0D
Welcome to the Sample App! Click on the link below to activate your accou=
nt:=0D
</p>=0D
=0D
<a href=3D"http://localhost:3000/account_activations/UIYKxqawmwt9Nx1okrfi=
gg/edit?email=3Drei%40mail.com">Activate</a>=0D
  </body>=0D
</html>=0D

----==_mimepart_5caece8d356db_22e63fd7b3711720292f1--

Redirected to http://localhost:3000/
Completed 302 Found in 316ms (ActiveRecord: 9.6ms)

該当のソースコード

Content-Transfer-Encoding: quoted-printable

期待する結果(railsチュートリアルの見本)

UserMailer#account_activation: processed outbound mail in 292.4ms
Sent mail to michael@michaelhartl.com (47.3ms)
Date: Mon, 06 Jun 2016 20:17:41 +0000
From: noreply@example.com
To: michael@michaelhartl.com
Message-ID: <f2c9222494c7178e@mhartl-rails-tutorial-3045526.mail>
Subject: Account activation
Mime-Version: 1.0
Content-Type: multipart/alternative;
 boundary="--==_mimepart_5755da6513e89_f2c9222494c71639";
 charset=UTF-8
Content-Transfer-Encoding: 7bit


----==_mimepart_5755da6513e89_f2c9222494c71639
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit

Hi Michael Hartl,

Welcome to the Sample App! Click on the link below to activate your account:

https://rails-tutorial-mhartl.c9users.io/account_activations/
-L9kBsbIjmrqpJGB0TUKcA/edit?email=michael%40michaelhartl.com

----==_mimepart_5755da6513e89_f2c9222494c71639
Content-Type: text/html;
 charset=UTF-8
Content-Transfer-Encoding: 7bit

<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <style>
      /* Email styles need to be inline */
    </style>
  </head>

  <body>
    <h1>Sample App</h1>

<p>Hi Michael Hartl,</p>

<p>
Welcome to the Sample App! Click on the link below to activate your account:
</p>

<a href="https://rails-tutorial-mhartl.c9users.io/account_activations/
-L9kBsbIjmrqpJGB0TUKcA/edit?email=michael%40michaelhartl.com">Activate</a>
  </body>
</html>

----==_mimepart_5755da6513e89_f2c9222494c71639--

該当のソースコード

Content-Transfer-Encoding: 7bit

試したこと

1.Userメイラーの生成
$ rails generate mailer UserMailer account_activation password_reset

2.テキストビューの変更
app/views/user_mailer/account_activation.text.erb

Hi <%= @user.name %>,

Welcome to the Sample App! Click on the link below to activate your account:

<%= edit_account_activation_url(@user.activation_token, email: @user.email) %>

3.HTMLビューの変更
app/views/user_mailer/account_activation.html.erb

<h1>Sample App</h1>

<p>Hi <%= @user.name %>,</p>

<p>
Welcome to the Sample App! Click on the link below to activate your account:
</p>

<%= link_to "Activate", edit_account_activation_url(@user.activation_token,
                                                    email: @user.email) %>

4.development環境のメール設定
config/environments/development.rb

  # Don't care if the mailer can't send.
  # メイラー
  config.action_mailer.raise_delivery_errors = true
  config.action_mailer.delivery_method = :test
  host = 'localhost:3000'
  config.action_mailer.default_url_options = { host: host, protocol: 'http' }
  config.action_mailer.perform_caching = false