Handle Rails 6.1 ActiveModel::Error#detail

Starting with Rails 6.1 ActiveModel::Error is an actual class:

> Active Model's errors are now objects with an interface that allows
> your application to more easily handle and interact with errors thrown
> by models. The feature[1] includes a query interface, enables more
> precise testing, and access to error details.
>
> [1] rails/rails#32313

As a result of this `resource.details` looks slightly different than in
previous versions and notably doesn't contain `:message` anymore, if
`object.errors.add(:something, message: 'some error')` was used to
specify a validation error.
https://github.com/rails/rails/blob/2a7ff0a5f54979b14b19f827c99295297dda411d/activemodel/lib/active_model/error.rb#L149

From the test suite:
```
note.errors.add(:title, message: 'has typos') if note.errors.key?(:title)
```

Rails 6.0:
```
{
  :title=>[{:error=>:invalid, :value=>"BAD_TITLE"},
           {:error=>{:message=>"has typos"}}],
  :quantity=>[{:error=>:less_than, :value=>109, :count=>100}]
}
```

Rails 6.1:
```
{
  :title=>[{:error=>:invalid, :value=>"BAD_TITLE"},
           {:error=>:invalid}],
  :quantity=>[{:error=>:less_than, :value=>100, :count=>100}]
}
```

The patch addresses this change by merging the error.message into its detail.

Author: coffeejunk

lib/jsonapi/rails.rb

@@ -55,8 +55,18 @@ def self.add_errors_renderer!
           model_serializer = JSONAPI::Rails.serializer_class(model, false)
         end
 
-        details = resource.messages
-        details = resource.details if resource.respond_to?(:details)
+        details = {}
+        if ::Rails::VERSION::MAJOR >= 6 && ::Rails::VERSION::MINOR >= 1
+          resource.map do |error|
+            attr = error.attribute
+            details[attr] ||= []
+            details[attr] << error.detail.merge(message: error.message)
+          end
+        elsif resource.respond_to?(:details)
+          details = resource.details
+        else
+          details = resource.messages
+        end
 
         details.each do |error_key, error_hashes|
           error_hashes.each do |error_hash|