fix for upload binary data via cpp bindings

[sasokol]

I created a new constructor with data size.

[bioothod]

Looks good to me, but it requires a test, please add one which calls some method with binary data and results you read back is invalid for current code and works good with your new code.

[sasokol]

Example:

...
char *data;
elliptics_client.write_prepare(key, "", 0,total);
uint64_t  chunk = buflen;
while ( length > 0 ) {
   ...
   if ( chunk > length ){
       chunk = length;
   } 

   FCGX_GetStr(data, chunk, request.in); // get a chunk of data from socket
   // argument_data file( data ); // In the storage bad data. But if I send to the socket a base64 encoded data then its's ok.
   argument_data file( data, chunk ); // This I use the fix.
   async_result = elliptics_client.write_plain(key, file, offset); //write the chunk into elliptick session
   offset += chunk;
....
}

[shindo]

// It isn't correct for binary data.

This comment is unnecessary as the constructor is not about binary data.

[shindo]

The pull request is not "fix" actually, as there's no bug. You just added "helper" constructor for binary data.

Without the "fix" you can already use the constructor from data_pointer_base:

argument_data file(argument_data::pointer_type::from_raw(data, chunk));

[sasokol]

OK, thank you. It works...
I not found the in docs and did not understand the headers.