Force all nexus_headers to be lower-case

pynxm.py

@@ -104,7 +104,7 @@ def _make_request(self, operation, endpoint, payload=None, data=None, headers=No
             timeout=30,
         )
         status_code = response.status_code
-        self.nexus_headers = {k: v for k, v in response.headers.items() if k.lower().startswith("x-")}
+        self.nexus_headers = {k.lower(): v for k, v in response.headers.items() if k.lower().startswith("x-")}
         if status_code not in (200, 201):
             if status_code == 429:
                 raise LimitReachedError(