Patch by Yuuji Yaginuma. [Fixes GH-1808]
In other classes, `FrozenError` will be raised if change the frozen
object.
In order to match the behavior, I think that `FrozenError` should
use in `OpenStruct`.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62271 b2dd03c8-39d4-4d8f-98ff-823fe69b080e