Fix specs on Rails 6 RC2 (#5109)

* Fix specs on Rails 6 RC2

`ActiveRecord::MigrationContext` now has a `schema_migration` attribute.
Ref: https://github.com/rails/rails/pull/36439/files#diff-8d3c44120f7b67ff79e2fbe6a40d0ad6R1018

* Use `media_type` instead of `content_type`

Before Rails 6 RC2, the `ActionDispatch::Response#content_type` method
would return only the media part of the `Content-Type` header, without any
other parts. Now the `#content_type` method returns the entire header -
as it is - and `#media_type` should be used instead to get the previous
behavior.

Ref:
- https://github.com/rails/rails/pull/36034
- https://github.com/rails/rails/pull/36854

* Use render template instead of render file

Render file will need the full path in order to avoid security breaches.
In this particular case, there's no need to use render file, it's ok to
use render template.

Ref: https://github.com/rails/rails/pull/35688

* Don't set `represent_boolean_as_integer` on Rails 6

* Update comments [ci skip]

test/rails_app/app/views/admins/sessions/new.html.erb

@@ -1,2 +1,2 @@
 Welcome to "sessions/new" view!
-<%= render file: "devise/sessions/new" %>
+<%= render template: "devise/sessions/new" %>

test/rails_app/config/application.rb

@@ -45,8 +45,8 @@ module RailsApp
       Devise::SessionsController.layout "application"
     end
 
-    # Remove this check once Rails 5.0 support is removed.
-    if Devise::Test.rails52_and_up?
+    # Remove the first check once Rails 5.0 support is removed.
+    if Devise::Test.rails52_and_up? && !Devise::Test.rails6?
       Rails.application.config.active_record.sqlite3.represent_boolean_as_integer = true
     end
   end

test/rails_app/config/boot.rb

@@ -6,8 +6,12 @@ end
 
 module Devise
   module Test
-    # Detection for minor differences between Rails 4 and 5, 5.1, and 5.2 in tests.
+    # Detection for minor differences between Rails versions in tests.
     
+    def self.rails6?
+      Rails.version.start_with? '6'
+    end
+
     def self.rails52_and_up?
       Rails::VERSION::MAJOR > 5 || rails52?
     end