Using `.open()` after default construction results in empty file

[Bidski]

If you default construct a zstr::ofstream object and then use .open() on the same object no data is written to the file.

This code snippet shows this issue. The file test.txt.gz will be empty at the termination of this program

#include <zstr.hpp>

int main(int argc, char** argv) {
    zstr::ofstream ofs{};

    ofs.open("test.txt.gz");
    ofs << "Use: " << argv[0] << " file" << std::endl
        << "Synopsis: Open `file` as a file stream object" << std::endl
        << "Stream Classes:" << std::endl
        << "  std" << std::endl
        << "  std_mask" << std::endl
        << "  strict_fstream" << std::endl
        << "Stream Types:" << std::endl
        << "  ifstream" << std::endl
        << "  ofstream" << std::endl
        << "  fstream" << std::endl
        << "Modes:" << std::endl
        << "  in=" << std::ios_base::in << std::endl
        << "  out=" << std::ios_base::out << std::endl
        << "  app=" << std::ios_base::app << std::endl
        << "  ate=" << std::ios_base::ate << std::endl
        << "  trunc=" << std::ios_base::trunc << std::endl
        << "  binary=" << std::ios_base::binary << std::endl;
    return 0;
}