depending on `ConfigOptions`,
the resolved value can be any of the following data types:
  * String (object)
  * Buffer
  * http.IncomingMessage

this signature is a minor change from previous releases:
  * string (primitive)
  * Buffer
  * http.IncomingMessage

the reason for changing String from a primitive to an object
is to add the ability to include additional object attributes.

specifically,
all 3 data types are now normalized to include the attribute:
  * "headers"
    * Object containing HTTP response headers:
      https://nodejs.org/api/http.html#http_message_headers