The http client application hangs in infinite loop in io_service::run, if server accept connection and do nothing. There are no reaction on closing socket at server side.
You can reproduce this using following steps:

• run server using command at separate terminal:

while true; do echo "" | nc -l 8091; done

• compile and run the program:

// main.cpp 
#include <iostream>
#include <boost/network/include/http/client.hpp>
int main ( int argc, char *argv[] )
{
  typedef boost::network::http::basic_client<
      boost::network::http::tags::http_async_8bit_tcp_resolve, 1, 1> 
      Client;
  Client client;
  Client::request req("http://localhost:8091");
  Client::response resp = client.get(req);
  std::cout << body(resp) << std::endl;
  return 0;
}

This program will work forever ( ever you terminate netcat ), and you can observe 100% CPU usage by this program at top utility.
system: Ubuntu 14.04 LTS 64bit
boost version: 1.54
cpp-netlib version: 0.11-devel (cpp-netlib-0.11.1-final also consists this isssue)

$ cat /usr/include/boost/version.hpp  | fgrep "#define BOOST_LIB_VERSION"
#define BOOST_LIB_VERSION "1_54"
$ git rev-parse HEAD
6e090b3d0b3c4f3b132432a6763594593cf7f365
$ git branch
* 0.11-devel
$ g++ --version
g++ (Ubuntu 4.8.2-19ubuntu1) 4.8.2

Jetty shows such server behaivior, when there are lack of memory.