This version of `rack` makes the request body optional. Omitting to
explicitly set `rack.input` on GET's will cause the request's header
verification to fail with:
```
NoMethodError:
  undefined method `read' for nil:NilClass
```
because there is no `request.body` to read the JWT token from.

See rack/rack#2018.

Ever since rack/rack#2018 `rack.input` has been
optional so we can't assume it's there and call `read` or `rewind` on it in
`Apipie::Extractor::Recorder`. Using safe navigation to call these methods
seems like the simplest approach because we want to look for
`rack.request.form_hash` instead in both the situation where `rack.input` is
missing, or it's empty.

Ever since rack/rack#2018 `rack.input` has been
optional so we can't assume it's there and call `read` or `rewind` on it in
`Apipie::Extractor::Recorder`. Using safe navigation to call these methods
seems like the simplest approach because we want to look for
`rack.request.form_hash` instead in both the situation where `rack.input` is
missing, or it's empty.

#939)

* Update how we use Rack::Utils to get status codes

As part of rack/rack#2137 (released in Rack 3) Rack
changed the values of `Rack::Utils::SYMBOL_TO_STATUS_CODE` so asking for
`:unprocessable_entity` now returns `nil`. The guidance has always been to use
`Rack::Util.status_code` (present in the API since Rack 1.1) so let's just do
that.

* Handle a missing `rack.input` in the env

Ever since rack/rack#2018 `rack.input` has been
optional so we can't assume it's there and call `read` or `rewind` on it in
`Apipie::Extractor::Recorder`. Using safe navigation to call these methods
seems like the simplest approach because we want to look for
`rack.request.form_hash` instead in both the situation where `rack.input` is
missing, or it's empty.

* Add rubocop-rspec_rails and get rubocop run to green

In rubocop/rubocop-rspec#1848 rubocop-rspec extracted
some cops to separate gems, and because we referenced one of them in our
rubocop_todo.yml the whole rubocop process wouldn't run because it doesn't like
config for cops it doesn't know about. We introducing the missing gem,
`rubocop-rspec_rails`, and fix the name in the config to let us run rubocop
again.

There is one infraction that exists in the source so we fix that to get us to a
green rubocop run.

* Only add rubocop-rspec_rails for ruby >-= 2.7

It's only available for 2.7+ and we still run this build on 2.6.