Convert buffer in range loops

fix #485

test/core/buffer_prefix.cpp

@@ -98,7 +98,7 @@ public:
         using boost::asio::buffer_size;
         std::string s;
         s.reserve(buffer_size(bs));
-        for(auto const& b : bs)
+        for(boost::asio::const_buffer b : bs)
             s.append(buffer_cast<char const*>(b),
                 buffer_size(b));
         return s;