rest: Set OAuth Content-Type header explicitly (apache#478)

Author: Fokko

pyiceberg/catalog/rest.py

@@ -293,9 +293,10 @@ def _fetch_access_token(self, session: Session, credential: str) -> str:
         # take scope from properties or use default CATALOG_SCOPE
         scope = self.properties.get("scope") or CATALOG_SCOPE
 
-        data = {GRANT_TYPE: CLIENT_CREDENTIALS, CLIENT_ID: client_id, CLIENT_SECRET: client_secret, SCOPE: scope}
-        # Uses application/x-www-form-urlencoded by default
-        response = session.post(url=self.auth_url, data=data)
+        data = {GRANT_TYPE: CLIENT_CREDENTIALS, CLIENT_ID: client_id, CLIENT_SECRET: client_secret, SCOPE: CATALOG_SCOPE}
+        response = session.post(
+            url=self.auth_url, data=data, headers={**session.headers, "Content-type": "application/x-www-form-urlencoded"}
+        )
         try:
             response.raise_for_status()
         except HTTPError as exc: