This change 'flattens' a TagHelperOutput when writing it out to an
HtmlTextWriter. This is a beneficial perf change because more often than
not the thing being written to is a ViewBuffer in Razor, which uses pooled
memory. This allows us to 'join' islands of pooled ViewBuffer memory back
into the main buffer instead of keeping them wrapped up in a
TagHelperOutput or TagHelperContent.
N. Taylor Mullen
Ryan Nowak