If you send an HTTP request with the header Expect set to the value 100-continue
and the server responds with an interim HTTP response with a status code of 100 and a continue handler
has been set using this method, then the handler will be called.
A map of all headers in the request, If the request contains multiple headers with the same key, the values
will be concatenated together into a single header with the same key value, with each value separated by a comma,
as specified here.