Allow inspection of ErrorExtensions for testing

[Follpvosten]

Description of the feature

ErrorExtensionValues is current write-only, as it doesn't implement Deref, the wrapped BTreeMap is private and it doesn't expose any method to get an existing extension.

This means to check for an expected extension, I have to convert it to a serde value first, for example:

let extensions = serde_json::to_value(error.extensions.unwrap()).unwrap();
let extensions = extensions.as_object().unwrap();
assert_eq!(extensions.get("code"), Some(&401.into()));

I'd suggest either implementing Deref<Target = BTreeMap<String, Value>> on ErrorExtensionValues, or alternatively just exposing a get(&self, key: &impl Borrow<String> + Ord) -> Option<&Value> method. This would make it much easier to check if a specific value has been set to an extension, without going through serde.

let extensions = error.extensions.unwrap();
assert_eq!(extensions.get("code"), Some(&401.into());

If you want to keep the extensions write-only for non-test code by default, you could feature-flag this impl or method.

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[github-actions]

This issue was closed because it has been stalled for 5 days with no activity.