lib/ostruct.rb: Revert "To use RuntimeError instead of FrozenError for old ruby versions."

This reverts commit 4cd1fc8b3559353069860eee90b1b5bade013917.

lib/ostruct.rb

@@ -170,8 +170,7 @@ class OpenStruct
     begin
       @modifiable = true
     rescue
-      exception_class = defined?(FrozenError) ? FrozenError : RuntimeError
-      raise exception_class, "can't modify frozen #{self.class}", caller(3)
+      raise FrozenError, "can't modify frozen #{self.class}", caller(3)
     end
     @table
   end

test/ostruct/test_ostruct.rb

@@ -66,16 +66,15 @@ class TC_OpenStruct < Test::Unit::TestCase
     o = OpenStruct.new(foo: 42)
     o.a = 'a'
     o.freeze
-    expected_error = defined?(FrozenError) ? FrozenError : RuntimeError
-    assert_raise(expected_error) {o.b = 'b'}
+    assert_raise(FrozenError) {o.b = 'b'}
     assert_not_respond_to(o, :b)
-    assert_raise(expected_error) {o.a = 'z'}
+    assert_raise(FrozenError) {o.a = 'z'}
     assert_equal('a', o.a)
     assert_equal(42, o.foo)
     o = OpenStruct.new :a => 42
     def o.frozen?; nil end
     o.freeze
-    assert_raise(expected_error, '[ruby-core:22559]') {o.a = 1764}
+    assert_raise(FrozenError, '[ruby-core:22559]') {o.a = 1764}
   end
 
   def test_delete_field