Fix mail tests on Rails 4.2

An update to mail broke some tests on Rails. Originally they were
unblocked here #31026 but Jeremy fixed them for Mail 2.7 in
0931e17e.

I wasn't able to cleanly backport this so I'm doing it manually. Fixes
the failing Rails 4.2 build on travis.

Gemfile.lock

@@ -134,10 +134,11 @@ GEM
       nokogiri
     loofah (2.0.3)
       nokogiri (>= 1.5.9)
-    mail (2.6.6)
-      mime-types (>= 1.16, < 4)
+    mail (2.7.0)
+      mini_mime (>= 0.1.1)
     metaclass (0.0.4)
     mime-types (2.99.3)
+    mini_mime (1.0.0)
     mini_portile2 (2.2.0)
     minitest (5.3.3)
     mocha (0.14.0)

actionmailer/test/base_test.rb

@@ -316,22 +316,21 @@ def welcome
 
   test "implicit multipart with attachments creates nested parts" do
     email = BaseMailer.implicit_multipart(attachments: true)
-    assert_equal("application/pdf", email.parts[0].mime_type)
-    assert_equal("multipart/alternative", email.parts[1].mime_type)
-    assert_equal("text/plain", email.parts[1].parts[0].mime_type)
-    assert_equal("TEXT Implicit Multipart", email.parts[1].parts[0].body.encoded)
-    assert_equal("text/html", email.parts[1].parts[1].mime_type)
-    assert_equal("HTML Implicit Multipart", email.parts[1].parts[1].body.encoded)
+    assert_equal(%w[ application/pdf multipart/alternative ], email.parts.map(&:mime_type).sort)
+    multipart = email.parts.detect { |p| p.mime_type == "multipart/alternative" }
+    assert_equal("text/plain", multipart.parts[0].mime_type)
+    assert_equal("TEXT Implicit Multipart", multipart.parts[0].body.encoded)
+    assert_equal("text/html", multipart.parts[1].mime_type)
+    assert_equal("HTML Implicit Multipart", multipart.parts[1].body.encoded)
   end
 
   test "implicit multipart with attachments and sort order" do
     order = ["text/html", "text/plain"]
     with_default BaseMailer, parts_order: order do
       email = BaseMailer.implicit_multipart(attachments: true)
-      assert_equal("application/pdf", email.parts[0].mime_type)
-      assert_equal("multipart/alternative", email.parts[1].mime_type)
-      assert_equal("text/plain", email.parts[1].parts[1].mime_type)
-      assert_equal("text/html", email.parts[1].parts[0].mime_type)
+      assert_equal(%w[ application/pdf multipart/alternative ], email.parts.map(&:mime_type).sort)
+      multipart = email.parts.detect { |p| p.mime_type == "multipart/alternative" }
+      assert_equal(%w[ text/html text/plain ], multipart.parts.map(&:mime_type).sort)
     end
   end
 
@@ -397,12 +396,12 @@ def welcome
 
   test "explicit multipart with attachments creates nested parts" do
     email = BaseMailer.explicit_multipart(attachments: true)
-    assert_equal("application/pdf", email.parts[0].mime_type)
-    assert_equal("multipart/alternative", email.parts[1].mime_type)
-    assert_equal("text/plain", email.parts[1].parts[0].mime_type)
-    assert_equal("TEXT Explicit Multipart", email.parts[1].parts[0].body.encoded)
-    assert_equal("text/html", email.parts[1].parts[1].mime_type)
-    assert_equal("HTML Explicit Multipart", email.parts[1].parts[1].body.encoded)
+    assert_equal(%w[ application/pdf multipart/alternative ], email.parts.map(&:mime_type).sort)
+    multipart = email.parts.detect { |p| p.mime_type == "multipart/alternative" }
+    assert_equal("text/plain", multipart.parts[0].mime_type)
+    assert_equal("TEXT Explicit Multipart", multipart.parts[0].body.encoded)
+    assert_equal("text/html", multipart.parts[1].mime_type)
+    assert_equal("HTML Explicit Multipart", multipart.parts[1].body.encoded)
   end
 
   test "explicit multipart with templates" do

actionmailer/test/test_helper_test.rb

@@ -38,7 +38,7 @@ def test_charset_is_utf_8
   end
 
   def test_encode
-    assert_equal '=?UTF-8?Q?This_is_=E3=81=82_string?=', encode('This is あ string')
+    assert_equal "This is あ string", Mail::Encodings.q_value_decode(encode("This is あ string"))
   end
 
   def test_read_fixture