Manual close of code_converter device does not flush data

Only when you manualy close `boost::iostream::code_converter` device the data are not flushed into the mapped device.

When you execute following fragment of the code, there is created empty file `log.txt`.
```CPP
void write_text_file()
{

boost::locale::generator gen;
typedef io::code_converter converter_device;
typedef io::stream converter_stream;

io::file_descriptor_sink file_sink;
converter_device dev;
converter_stream stream;

dev.imbue(gen("C.UTF-8"));

file_sink.open("log.txt", std::ios_base::out);

dev.open(file_sink);
stream.open(dev);

stream << L"Write UTF-8 string\n"
<< L"Zapiš UTF-8 řetězec";

stream.close();
dev.close();
file_sink.close();

cout << "file_sink.is_open = " << file_sink.is_open() << "\n"
<< "dev.is_open = " << dev.is_open() << "\n"
<< "stream.is_open = " << stream.is_open();
}
```

From the following output of the program is see the converter device is still open:
```
file_sink.is_open = 0
dev.is_open = 1
stream.is_open = 0
```
The reason is the method `close` has the input argument `which` for identification to close input or output stream. The default value is bitwise or of constant `in | out`.

```CPP
class code_converter
: protected code_converter_base
...
public:
void close(BOOST_IOS::openmode which = BOOST_IOS::in | BOOST_IOS::out )
{ impl().close(which); }
```

Private method `impl()` returns instance of `code_converter_impl` where the method `close` is overladed and implemented as follows:

```CPP
// Contains member data, open/is_open/close and buffer management functions.
template
struct code_converter_impl {
void close(BOOST_IOS::openmode which)
{
// BUG: Condition block is called only when which is either in or out but cannot be
// combination of both
if (which == BOOST_IOS::in && (flags_ & f_input_closed) == 0) {
...
}
if (which == BOOST_IOS::out && (flags_ & f_output_closed) == 0) {
....
);
}
}
```
When the `which` argument has the value `which = in | out` which is the default value provided by `code_convert::close` no condition block is called at all.
The result is the device is not closed and data in the buffer are lost.