fix(poller): perform case-insensitive lookup for etag header and upda…

[jose-gil]

#136

Describe the bug

In Poller.swift, the ETag header was accessed directly from HTTPURLResponse.allHeaderFielsusing key indexing: if let newTag = httpResponse.allHeaderFields["Etag"] as? string.

Becase allHeaderFields returns a standard dictionary ([AnyHashable: Any]), indexing it with ["Etag"] performs a case-sensitive lookup. This is also confirmed in the documentation in the discussion section (https://developer.apple.com/documentation/foundation/httpurlresponse/allheaderfields).

About the changes

According to the HTTP specification (RFC 7230), HTTP header field names are case-insensitive. In production environments-especially over HTTP/2 or behind proxies/CDNs that lowercase header names (e.g., etag instead of Etag) httpResponse.allHeaderFields["Etag"] returns nil. As a result, the SDK fails to capture and store the updated etag, preventing If-None-Match caching from working properly.

Why existing unit tests missed this bug

The existing unit test testLowerCaseEtagResponseHeader appeared to pass becase it instantiate HTTPURLResponse directly via: HTTPURLResponse(url: url, statusCode: 200, httpVersion: nil, headerFields: ["etag": "..."]).

Apple's HTTPURLResponse initializer canonicalizes dictionary keys in headerFields upon creation. This masked the bug in unit testing becase the mock response keys were normalized, hiding the fact that direct dictionary subscribing on allHeaderFields["Etag"] is case-sensitive.

Proposed Fix

• Use valuer(forHTTPHeaderFields:) in Poller.swift:
  Replace direct dictionary lookup with Foundation's standard case-insensitive header lookup method (https://developer.apple.com/documentation/foundation/httpurlresponse/value(forhttpheaderfield:)).

• Fix Unit Tests (PollerTests.swift):
  Subclass/mock HTTPURLResponse in test suites so header keys retain their exact case without Foundation's automatic key normalization.

[sighphyre]

Cheers, thanks for fixing this. Looks small and safe