lib/ostruct.rb: Use FrozenError instead of RuntimeError.

Patch by Yuuji Yaginuma. [Fixes rubyGH-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

lib/ostruct.rb

@@ -156,7 +156,7 @@ def modifiable? # :nodoc:
     begin
       @modifiable = true
     rescue
-      raise RuntimeError, "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

@@ -66,15 +66,15 @@ def test_frozen
     o = OpenStruct.new(foo: 42)
     o.a = 'a'
     o.freeze
-    assert_raise(RuntimeError) {o.b = 'b'}
+    assert_raise(FrozenError) {o.b = 'b'}
     assert_not_respond_to(o, :b)
-    assert_raise(RuntimeError) {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(RuntimeError, '[ruby-core:22559]') {o.a = 1764}
+    assert_raise(FrozenError, '[ruby-core:22559]') {o.a = 1764}
   end
 
   def test_delete_field