Add `File.absolute_path?` (#2198)
Add `File.absolute_path?` (#2198)

In order to check whether a path is absolute or not in a portable way.

[Feature #15868]
eregon/rubyspec,nobu/rubyspec,ruby/spec,ruby/spec,ruby/spec,nobu/rubyspec,eregon/rubyspec,eregon/rubyspec,nobu/rubyspec