* struct.c (rb_struct_set): return assigned value from setter method
  rather than struct object. [Bug #9353] [ruby-core:59509]

* test/ruby/test_struct.rb (test_setter_method_returns_value): add test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44501 b2dd03c8-39d4-4d8f-98ff-823fe69b080e