handle application-xml mimetype as HTML not XLS

We've encountered XHTML files being served with an `application-xml` mimetype. Messytables treated these files as XLS files, and then raised an error about:

```
Unsupported format, or corrupt file: Expected BOF record; found \
```

This pull request makes messytables treat `application-xml` files as HTML files not XLS files.

As far as I can tell, all the tests continue to pass.