Fix webhook requests for headers with a symbol key

[phylor]

Description

Webhooks fail when there is a header with a symbol as a key:

ShopifyAPITest::Webhooks::WebhookRequestTest#test_with_symbol_headers:
NoMethodError: undefined method `sub' for :clearance:Symbol
Did you mean?  stub

This happens e.g. when using Clearance, which adds :clearance as a header, containing the Clearance session. This means that webhooks will always fail as soon as you include Clearance into the project. Its middleware always adds that header, even if there is no authenticated request.

How has this been tested?

Test case within this PR.

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.

[paulomarg]

Looks good, thank you for adding a test as well!

Seems we got a couple of rubocop failures, could you please address those so we can merge?

[phylor]

Seems we got a couple of rubocop failures, could you please address those so we can merge?

@paulomarg Fixed!

[paulomarg]

Sorry for the delay in getting back to you @phylor, but it seems like we still have one failure when running the sorbet checks (bundle exec srb tc). Can you make sure that's passing so we can merge the PR? Thank you!

[phylor]

Sorry for the delay in getting back to you @phylor, but it seems like we still have one failure when running the sorbet checks (bundle exec srb tc). Can you make sure that's passing so we can merge the PR? Thank you!

No problem, sorry for the failures 🙂 Rebased on main and sorbet is green for me locally.

[phylor]

@paulomarg Sorry for the trouble. We're now getting Rubocop issues not relevant to my changes. Shall I still fix them in this PR?

[paulomarg]

No need, we'll fix those! Thank you.