`FileUtils#install` methed raises an unexpected `TypeError`, when
called with `mode:` option which has `"X"`.

```
$ ruby -rfileutils -e 'FileUtils.install("tmp/a", "tmp/b", mode: "o+X")'
/opt/local/lib/ruby/2.7.0/fileutils.rb:942:in `directory?': no implicit conversion of File::Stat into String (TypeError)
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:942:in `block (3 levels) in symbolic_modes_to_i'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:933:in `each_char'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:933:in `each'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:933:in `inject'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:933:in `block (2 levels) in symbolic_modes_to_i'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:931:in `each'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:931:in `each_slice'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:931:in `block in symbolic_modes_to_i'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:926:in `each'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:926:in `inject'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:926:in `symbolic_modes_to_i'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:973:in `fu_mode'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:883:in `block in install'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:1588:in `block in fu_each_src_dest'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:1604:in `fu_each_src_dest0'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:1586:in `fu_each_src_dest'
	from /opt/local/lib/ruby/2.7.0/fileutils.rb:877:in `install'
	from -e:1:in `<main>'
```

In spite of that `symbolic_modes_to_i` considers the `File::Stat`
`path` case at the beginning, in `"X"` case, `path` is passed to
`FileTest.directory?` method which requires a `String`.  In such
case, the mode in `path` should be examined instead.