Add extensions for TagHelperOutput to copy from TagBuilder.

- Tested out Merge, MergeAttributes, PullPrefixedAttributes and RestoreBoundHtmlAttribute.

#1319
despite all the comments, I still don't understand the use case for this method. is the issue case of the attribute name e.g. users using un-traditional casing (why help them :smiley_cat:), spacing in the original value, something else?