Properly deal with non-empty "Content-Transfer-Encoding"

`header['content-transfer-encoding']`   is  an   `Array`  &   cannot  be
converted to lower-case as-is:

```
header: { 'content-type': [ 'application/octet-stream' ],
  'content-disposition': [ 'form-data; name="file"; filename="tata"' ],
  'content-transfer-encoding': [ 'base64' ] }
```

If accepted, could this PR cause a version bump into npmjs.org?