Loading from a document then saving leads to a corrupt docx file

Tried with a few different docx files:

```use docx::DocxFile;

fn main() {
    let docx = DocxFile::from_file("test.docx").unwrap();
    let mut docx = docx.parse().unwrap();
    docx.write_file("test_2.docx").unwrap();
}
```

creates 1.  A file that is much, much smaller than the original document and 2. Creates a file that is unreadable/corrupt/unusable by Microsoft Word.