Case Insensitivity Issue with #dig Method on Response Headers

[vitali-semenyuk]

Basic Info

• Faraday Version: 2.9.0
• Ruby Version: 3.1.2

Issue description

I've encountered an issue with the case-insensitive Hash which is used as a response headers container. While the case insensitivity works as expected when accessing headers using #[], it fails when using the #dig method. Rack handles this properly: https://github.com/rack/rack/blob/8c73aefcc7085c71bdfe6c1ec867f126ede34124/lib/rack/headers.rb#L131-L133

Steps to reproduce

require 'faraday'

response = Faraday.get('http://httpbingo.org')

pp response.headers.dig('via') # "1.1 fly.io"
pp response.headers.dig('Via') # nil

[olleolleolle]

Cool find, would you like to try making a PR?

Can it be done without changing all adapters? Probably!