feat: Only override content-type if it is none

* Only override content-type if it is none

Author: rmkeezer

ibm_cloud_sdk_core/base_service.py

@@ -292,7 +292,8 @@ def prepare_request(self,
 
         if data and isinstance(data, dict):
             data = remove_null_values(data)
-            headers.update({'content-type': 'application/json'})
+            if headers.get('content-type') is None:
+                headers.update({'content-type': 'application/json'})
             data = json_import.dumps(data)
         request['data'] = data