Issue title: 6.1.0.rc1 Strict loading assocations doesn't work with ActiveStorage
 Issue body: I don't know if there is already a tracker for this, or this is planned in a more future release but I'll just leave it here.

### Steps to reproduce
```ruby
# app/models/supplier.rb
class Supplier < ApplicationRecord
  has_one_attached :logo, strict_loading: true
end

# app/controllers/suppliers_controller.rb
class SuppliersController < ApplicationController
  def index
    suppliers = Supplier.all
    # suppliers = Supplier.includes(:logo_attachment) # When eager loaded

    render json: suppliers.as_json(include: :logo)
  end
end
```

### Expected behavior
It should raise `ActiveRecor38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5StrictLoadingViolationError` error when not eager loaded.

### Actual behavior
I got `unknown keyword: :strict_loading` error instead.

### System configuration
**Rails version**:
```ruby
gem 'rails', '~> 6.1.0.rc1'
```

**Ruby version**:
```
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin19]
```

 Comments: 
 Comment 0: After this change, writing
```ruby
has_one_attached :logo
```
is equal to 
```ruby
has_one_attached :logo, strict_loading: false
```
which doesnt always represent user's intent.

The problem was introduced by setting the default value [here](https://github.com/dcangulo/rails/blob/1db02b3896501ab4925e017538ee69f65acbc0ba/activestorage/lib/active_storage/attached/model.rb#L50) when user does not provide one, and doesn't taking into account the `strict_loading_by_default` setting in such case.

Consider this example

```ruby
class Avatar
  has_one_attached :logo
end

class AvatarTest < ActiveSupport38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5TestCase
  test "n+1 queries" do
    Avatar.strict_loading_by_default = true
    # N+1 query doesn't raise ActiveRecor38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5StrictLoadingViolationError error
  end
end
```

This is counterintuitive and doesn't match `has_one` behaviour.
`strict_loading` option should be appended [here](https://github.com/dcangulo/rails/blob/1db02b3896501ab4925e017538ee69f65acbc0ba/activestorage/lib/active_storage/attached/model.rb#L70) only when user explicitly specify one.

{'source': 'GithubIssues', 'repo_name_with_owner': 'rails/rails', 'issue_id': 743042929, 'repo_stars': 55368.0}