fix http client body content (cpp-netlib#760)

Author: chenzhaoyu

boost/network/protocol/http/client/connection/async_protocol_handler.hpp

@@ -389,7 +389,7 @@ struct http_async_protocol_handler {
     partial_parsed.append(part_begin, part_begin + bytes);
     part_begin = part.begin();
     if (check_parse_body_complete()) {
-      callback(boost::asio::error::eof, bytes);
+      callback(boost::asio::error::eof, 0);
     } else {
       delegate_->read_some(
           boost::asio::mutable_buffers_1(part.data(), part.size()), callback);