Add sort_by! spec for single-element array.
Add sort_by! spec for single-element array.

This case broke in JRuby's specialized "packed" arrays and there
is no other test for it.

nobu/rubyspec,sgarciac/spec,nobu/rubyspec,kachick/rubyspec,kachick/rubyspec,kachick/rubyspec,sgarciac/spec,ruby/spec,ruby/spec,ruby/spec,eregon/rubyspec,eregon/rubyspec,sgarciac/spec,nobu/rubyspec,ruby/rubyspec,eregon/rubyspec,ruby/rubyspec