`URI::FTP#normalize` returns incorrect (?) value if original path is empty

I don't know how common this case is, but I did run into it accidentally while testing some code.

u = URI("ftp://ftp.example.com") # Note that there's no slash at the end
u.normalize.to_s # => "ftp://ftp.example.com/%2F"

I would expect the result to be "ftp://ftp.example.com/" or possibly the same as the original string, "ftp://ftp.example.com". As best as I can tell from RFC 1738, Section 3.2, "ftp://ftp.example.com" and "ftp://ftp.example.com/" are equivalent and refer to whatever the "default" directory is for FTP access, while "ftp://ftp.example.com/%2F" refers to the root directory.

This seems to be caused by a combination of two things. First, this code in URI::Generic#normalize!calls set_path('/') if the path is empty:
https://github.com/ruby/uri/blob/52077e9b07c555de6ad7ee74663b988fa38ca545/lib/uri/generic.rb#L1341-L1343

Second, URI::FTP overrides #set_path so that a leading slash in the argument is percent-encoded and appended to an unencoded slash: https://github.com/ruby/uri/blob/52077e9b07c555de6ad7ee74663b988fa38ca545/lib/uri/ftp.rb#L245-L247

I didn't see any tests for normalizing FTP URIs, so I'm not sure if this behavior is intended.

Start with URI::Generic#normalize! in lib/uri/generic.rb and URI::FTP#set_path in lib/uri/ftp.rb, using the linked lines as entry points. Check RFC 1738 and add focused normalization tests for an FTP URI with an empty path. Done means the behavior is decided and the result no longer incorrectly encodes the default directory as %2F.