Change MiddlewareFactory to type activate IMiddleware as a fallback

[davidfowl]

• This eases the transition and learning when you discover there's an interface for middleware.
• Inspired by https://stackoverflow.com/questions/48394756/no-service-for-type-has-been-registered/ /cc @guardrex

[khellang]

This means you're able to activate IMiddleware instances without them being registered in the container first, i.e. by just doing Use<TMiddleware> where TMiddleware implements IMiddleware?

[davidfowl]

@khellang yap