Fix using rack v3

[RichardVickerstaff]

These changes get the Gem running locally again, with a few tune ups and one actual fix:

• lobster is no longer part of Rack in newer versions. We can still refer to it via Rackup. Since it's only used for the specs, we can make this a dev dependency.
• Requiring logger is important for specs. This will have been working elsewhere because logger is a fairly normal thing to include, and in a Rails-y environment it'll be zeitwerk'd.
• GET requests to not have 'rack.input' in later versions of rack, so double check it's available before trying to rewind.

[PChambino]

This is only a dev/test dependency, so it can be moved to add_development_dependency.

[shamess]

I have to admit some ignorance when working with rack, especially when it's being used inside a gem.

What's the purpose of the ruby/config.ru rackup configuration? Is this ever expected to be run by anyone? Is it just a demo (duplicating the content of the README)?

[PChambino]

ruby/config.ru is only being used by that integration_spec.rb file. It is just for testing. It is not supposed to be used by any consumers of this gem. It could probably be moved into the spec directory for clarity.