feat: rack 2 and 3 compat

[YOU54F]

Building on #73

This PR provides backwards compatibility for Rack 2, preserving the existing behaviour for Rack 2 users, whilst allowing Rack 3 users to still utilise this project.

#73 would fail on earlier versions of Ruby due to the bundler version being updated. We can relax that restriction to allow users on older and newer versions to proceed

Test runs on Ruby 2.7 through to 3.3, with Rack 2 & 3.

One test is failing with Rack 2, as the code stands in master

Failures:

  1) RackReverseProxy::Middleware as a rack app responds with 404 when the path is not matched
     Failure/Error: get "/"

     NoMethodError:
       undefined method `has_key?' for []:Array
     # ./spec/rack/reverse_proxy_spec.rb:764:in `block (3 levels) in <top (required)>'

master...pact-foundation:rack-reverse-proxy:refs/heads/master-ci