Some text decoration attributes replace each other instead of merging

[webwarrior-ws]

Example:

text := 'Hello world' asRopedText.
text
	attributes: {
		BlTextDecorationAttribute new underline.
		BlTextDecorationAttribute new lineThrough.
	} 
	from: 1
	to: 5.

Expected behavior:

Hello is underlined and strikethrough.

Actual behavior:

Hello is just strikethrough.

My thoughts

From debugging it seems that this happens because both attributes have the same type.