Deprecate implicit AD::Response splatting and Array conversion

Deprecate implicit Array conversion for Response objects. It was added (using `#to_ary`) so we could conveniently use implicit splatting:
```ruby
status, headers, body = response
```
But it also means `response + response` works and `[response].flatten` cascades down to the Rack body. Nonsense behavior. Instead, rely on explicit conversion and splatting with `#to_a`:
```ruby
status, header, body = *response
```
Should probably remove this. Some code, like `Rack::ContentLength`, checks whether `body.respond_to? :to_ary` rather than calling it and using its result (or respecting a `nil` return value).