ruby.h: fix rb_scan_args_trail_idx

	* include/ruby/ruby.h (rb_scan_args_trail_idx): fix the case both
	  of optional and rest arguments are defined.
	  [ruby-core:82427] [Bug #13830]

	* include/ruby/ruby.h (rb_scan_args_n_trail): ditto.
	non-keywords hash

	* class.c (rb_scan_args), include/ruby/ruby.h (rb_scan_args_set):
	  return non-keywords elements only in the last hash when keyword
	  arguments are extracted from it, as well as methods defined in
	  ruby level.  [ruby-core:82427] [Bug #13830]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59811 b2dd03c8-39d4-4d8f-98ff-823fe69b080e