For example you can iterate over all headers in a response object this way:

for (const auto& header : response.headers())
	sHeaders += header.first + U(": ") + header.second;

I want to read request headers when handling a HTTP requests.

It's the same, e.g.

web::http::http_request request;
...
string_t sHeaders;
for (const auto& header : request.headers())
	sHeaders += header.first + U(": ") + header.second;