serializer should provide an interface to capture and cache HTTP responses

[ashtum]

This is useful when a server wants to cache static responses specially when they need compression.

[vinniefalco]

I dont know if serializer should provide that API but we need to determine if it is possible to write this function:

template< class Body >
std::string
serialize( serializer& sr, Body&& body );

given the current API. I think this should go into an example.