HTTP client body data sinks copy data

[xemul]

Recently, the sink impls were fixed not to crash in case caller tries to use zero-copy writing (52eb05829b271984dd4c79dae940057e70c13c2c), but the chunked and content-length sinks both use fallback implementation of put(net::packet) methods. The fallback, in turn, calls the put(temporary_buffer<>) one which, in case of http body sinks, both call _out.write(buf.get(), buf.size()) and copy the buffer data into socket stream