patch 8.1.2166: rubyeval() not tested as a method

Problem:    Rubyeval() not tested as a method.
Solution:   Change a test case.
diff --git a/src/testdir/test_ruby.vim b/src/testdir/test_ruby.vim
index f679bbd..8f83a30 100644
--- a/src/testdir/test_ruby.vim
+++ b/src/testdir/test_ruby.vim
@@ -46,7 +46,7 @@
   " Check that movement after setting cursor position keeps current column.
   normal j
   call assert_equal([2, 6], [line('.'), col('.')])
-  call assert_equal([2, 5], rubyeval('$curwin.cursor'))
+  call assert_equal([2, 5], '$curwin.cursor'->rubyeval())
 
   call assert_fails('ruby $curwin.cursor = [1]',
         \           'ArgumentError: array length must be 2')