Serialize textarea value correctly

[lydell]

Currently, Html.textarea [ Html.Attributes.value "my value" ] [] is serialized as HTML as <textarea value="my value"></textarea>. However, that results in an empty textarea in HTML. The correct serialization is <textarea>my value</textarea>.

For full completeness, elm/virtual-dom also needs to support going the other way: When virtualizing, it should create vdom for Html.textarea [ Html.Attributes.value "my value" ] [], not Html.textarea [] [ Html.text "my value" ]. My to-be-released fork of elm/virtual-dom does this.

[dillonkearns]

Makes sense, thank you for the issue!

The HTML serialization code here is originally from the elm-test codebase, and I try to keep it in lockstep as much as possible. For example, I made a fix a while ago and made a PR over there elm-explorations/test#212. Would be nice to do the same for this.