feat(session): Optimize header handling in session requests

Removed redundant conversion of headers to dict for requests.

unshackle/core/session.py

@@ -584,11 +584,12 @@ class RnetSession:
         if rnet_method is None:
             raise ValueError(f"Unsupported HTTP method: {method}")
 
+        # Convert headers to standard dict once to resolve PyO3 CaseInsensitiveDict rejection.
+        if kwargs.get("headers") is not None:
+            kwargs["headers"] = dict(kwargs["headers"])
+
         # Skip retry for non-allowed methods
         if method_upper not in self.allowed_methods:
-            if "headers" in kwargs and kwargs["headers"] is not None:
-                kwargs["headers"] = dict(kwargs["headers"])
-
             raw_resp = client.request(rnet_method, url, **kwargs)
             return RnetResponse(raw_resp)
 
@@ -597,9 +598,6 @@ class RnetSession:
 
         for attempt in range(self.max_retries + 1):
             try:
-                if "headers" in kwargs and kwargs["headers"] is not None:
-                    kwargs["headers"] = dict(kwargs["headers"])
-
                 raw_resp = client.request(rnet_method, url, **kwargs)
                 response = RnetResponse(raw_resp)
                 if response.status_code not in self.status_forcelist: