Content-Type is a standard HTTP header and is already a string (because headers are key-value pairs). There are no double quotes in them. I think the error is misleading you. Unless you're trying to parse the response by hand the
Content-Type is being read properly and it is just how the error message is being shown. ADF supports JSON as it is the most common API format.
Can you provide us information on how you're consuming the API call including any settings you're using.