download_ramlegacy() still prompts even when user sets overwrite = TRUE

[cboettig]

download_ramlegacy(overwrite=TRUE) should result in the function just downloading a new file, but instead, it still asks if the user wants to overwrite it. The function should only be interactive if overwrite is set explicitly to "ask", since it doesn't make sense to ask a user whether to overwrite if the user has set overwrite already when calling the function.

This is a duplicate of #4 and important for use in Rmd notebooks, etc, which count as being in "interactive" sessions