Fix prepare_single_hop_path_and_storage_options
Fix `_prepare_single_hop_path_and_storage_options`:
- Do not pass HF authentication headers and HF user-agent to non-HF HTTP URLs
- Do not overwrite passed `storage_options` nested values:
  - Before, when passed 
    ```DownloadConfig(storage_options={"https": {"client_kwargs": {"raise_for_status": True}}})```, 
    it was overwritten to 
    ```{"https": {"client_kwargs": {"trust_env": True}}}```
  - Now, the result combines both: 
    ```{"https": {"client_kwargs": {"trust_env": True, "raise_for_status": True}}}```