boost::span rvalue is not supported by asio::buffers

I have a function that returns a buffer as a span to read the data from socket:

```cpp
boost::span getRecvBuffer();
```

When I call `tcp::socket::async_receive(asio::buffer(getRecvBuffer()))`,

```cpp
socket_.async_receive(asio::buffer(getRecvBuffer()), [](auto&&...) {});
```

... I get an error:

```
error: no matching function for call to ‘boost::asio::mutable_buffer::mutable_buffer(const boost::asio::const_buffers_1&)’
```

The weird thing is that following code compiles:

```cpp
auto b1 = asio::buffer(getRecvBuffer());
```

Following code compiles too:

```cpp
auto buf = client.socketRecvBuffer_.recvBuffer();
socket_.async_receive(asio::buffer(buf), [](auto&&...) {});
```

It seems in order to create a `mutable_buffer`, I need a lvalue for the buffer, which makes sense in case of `std::vector`, but it does not make sense for `boost::span`.