Adds an override flag to change behaviour when Jets is behind an ELB running Jets Server

[TeddiO]

• This is a 🙋‍♂️ feature or enhancement.
• This is a 🧐 documentation change.

• I've added tests (if it's a bug, feature or enhancement)
• I've adjusted the documentation (if it's a feature or enhancement)
• The test suite passes (run bundle exec rspec to verify this)

Summary

This PR adds an override flag (JETS_SERVER_ELB_OVERRIDE) so if jets server is being fed traffic through an ELB, we get the same behaviour as if we were hitting it locally or through another load balancer.

Context

As part of some internal work going on where I work, there's been a need to create a stronger feedback loop with a copy of various services, including one that uses Jets. These services can be fed through an ELB (typically an ALB) and will generally work as they would as if in a production environment however, Jets appears to diverge in its behaviour if we're behind an ELB -

https://github.com/boltops-tools/jets/blob/1ccd80b49d22bfdd53fa22f998b6faae37d8fe33/lib/jets/controller/middleware/local.rb#L71-L74

Returning false here resolves the issue and gets our intended behaviour. Unfortunately, I don't have enough context as to why this is being done and I don't want to risk potentially breaking anyone that might be using behaviour around this so I've added it in as a flag instead.

I'm not a Ruby developer by trade so please feel free to give pointers if there's a better way!

How to Test

Set up Jets to work behind an ELB and running in jets server. Without JETS_SERVER_ELB_OVERRIDE you'll get a 500 error (as well as a stack trace if you're running in a debug mode). JETS_SERVER_ELB_OVERRIDE being set to any value (recommendation 1) will override this logic and have it functioning as intended.

Version Changes

A patch bump (e.g. 3.0.14) is recommended for a bump in version.

[tongueroo]

Made some slight adjustments in #586. The flag is called JETS_ELB