* string.c (sym_equal): use rb_obj_equal().
  rb_obj_equal() is specially optimized in
  opt_eq_func()@vm_insnhelper.c.
  This fix is made from this discussion:
  https://www.omniref.com/ruby/2.1.4/symbols/Symbol/%3D%3D#line=8361.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48369 b2dd03c8-39d4-4d8f-98ff-823fe69b080e