Unexpected Error due to Duplicate `Content-Type` Headers in `cwltool`

When I run the following command with the latest version of `cwltool`:

```bash
$ cwltool https://sandbox.zenodo.org/record/1016630/files/trimming_and_qc.cwl --help
```

I encounter the error below:

```bash
While fetching https://sandbox.zenodo.org/record/1016630/files/trimming_and_qc.cwl, got content-type of 'application/octet-stream, application/octet-stream'. Expected one of ['text/plain', 'application/json', 'text/vnd.yaml', 'text/yaml', 'text/x-yaml', 'application/x-yaml', 'application/octet-stream'].
```

I thought this might be related to the fix I provided in the past at:

https://github.com/common-workflow-language/cwltool/pull/1622

However, upon closer inspection, I noticed that the content-type is duplicated: `application/octet-stream, application/octet-stream`.

I fetched the actual headers using `curl`, and observed:

```bash
$ curl -D - https://sandbox.zenodo.org/record/1016630/files/trimming_and_qc.cwl
...
Content-Type: application/octet-stream
...
Content-Type: application/octet-stream
...
```

It seems there are two Content-Type lines.

I suspect the code around:
https://github.com/common-workflow-language/schema_salad/blob/e16612a7cf2d6cd9138aafdec20958452be3b611/schema_salad/fetcher.py#L75

might be related to this issue, but I'm not sure about the exact solution. Could you please look into this?