prettybib does not print strings with \url{} correctly

[nickng]

Input bibtex

@article{abcd,
    title  = "Title",
    author = "Doe, John",
    howpublished = {\url{http://example.com/}},
    notes = "\url{https://google.com/}",
}

Output bibtex

@article{abcd,
  title        = "Title",
  author       = "Doe, John",
  howpublished = {\url{http://example.com/}},
  notes        = "\urlhttps://google.com/",
}

The {} of the url macro should not be removed in the notes field. It should either be converted to a {} quote or kept as a special case for "" quote.