Otherwise, the path could be considered part of the host or port.

This is better than modifying the path to make it absolute when
a host or port is set.  We could also raise for invalid paths
when a host or port is set using check_path, but that results
in weird errors, and won't catch issues (such as ftp allowing a
relative path).

Fixes [Bug #19916]

https://github.com/ruby/uri/commit/ac32aa005b