HTTPClient: log actual response status code (#2487)

We were currently logging the status code retrieved from `ETagManager`,
which made it seem like etags weren't working.

Example:
> API request completed: GET /v1/subscribers/identify (200)

With this change, we now log the actual response status code so we can
verify the server is returning the correct status code:
> API request completed: GET /v1/subscribers/identify (304)

Sources/Networking/HTTPClient/HTTPStatusCode.swift

@@ -93,3 +93,13 @@ extension HTTPStatusCode {
     }
 
 }
+
+extension URLResponse {
+
+    var httpStatusCode: HTTPStatusCode? {
+        guard let response = self as? HTTPURLResponse else { return nil }
+
+        return .init(rawValue: response.statusCode)
+    }
+
+}