Missing 'filename' param while downloading a document

According to offical API docs - it is possible to pass optional param, called `filename`, which will use custom filename in the Content-Disposition header.
See https://crocodoc.com/docs/api/#dl-doc

However, this functionality is missing in current version of `crocodoc-python`.

This PR fixes it. It also fixes a misspelled variable in `crocodoc.download.document` function.