Support binding text/plain request body to action string args

[DamianEdwards]

So one can easily post "text/plain" content and get it in an action method with a string arg, e.g.

// POST /string-post HTTP/1.1
// Content-Type: text/plain
// Hello, World!
[HttpPost("string-post")]
public IActionResult StringPost(string payload)
{
    // payload contains "Hello, World!"
    ...
}

[kichalla]

Added a sample to Entropy aspnet/Entropy@7a4ca4b