Make TextareaElement's withContent() not append & withAttribute('value') set content

[bjuppa]

Textareas have their "value" set in content, not in attribute. When adding content to a textarea it should not be appended, it should replace any current content.

This could be done by overriding withContent() and have it call withDefaultContent().

[bjuppa]

Also, if calling withAttribute('value') on TextareaElement the developer's intention is probably to set content, not the value attribute (it is not allowed on textarea anyway).

This would help if calling setInputAttribute('value') on a control block including a textarea.