http://www.gutenberg.lib.md.us mirror working

[jrdnbradford]

gutenberg_get_mirror has the following:

 # ...
  if (parsed$domain == "www.gutenberg.lib.md.us") {
    # Broken mirror. PG has been contacted. For now, replace:
    parsed$domain <- "aleph.gutenberg.org" # nocov
  }
# ...

The http://www.gutenberg.lib.md.us mirror seems to be up and running again. However, I also noticed this mirror is not listed on the official mirrors page.

There are a few things I think could be worth doing based on this:

1. Remove the above code so that this mirror can be used.
2. Document as part of the gutenberg_get_all_mirrors function that it retrieves officially listed mirrors and that other mirrors may exist.
3. Perhaps add an unexported function to test whether or not a particular mirror is indeed a mirror and is working. Maybe by attempting to request GUTINDEX.zip from the mirror, and providing some sort of feedback? Maybe this should be a different PR than 1 and 2 if we want to do this.

@jonthegeek let me know what you think here.

[jonthegeek]

Yeah, go ahead and do 1 and 2, and write up 3 as a separate issue.