Resolves a bunch of duplication between TagHelperContent and
BufferedHtmlContent, plus adds a few more convenience overloads we don't
have in one of the two places.
This is needed because a builder may have an optimized path for an
unencoded string. There's also no 'common' encoded string implementation
so it's much more straightforward to put this on the interface.
Common interface for things that allow appending content (TagBuilder,
BufferedHtmlContent). We want to be able to expose this from APIs for
users to add content.
See discussion: https://github.com/aspnet/Mvc/issues/3087
Chris R
Doug Bunting
Pranav K
Ryan Nowak
sornaks