Missing setters from public collection package

[blotus]

Hello,

I was trying to write a custom body processor, and I realized that the interfaces (except for Map) in the collection package do not expose any setter (contrary to the internal collections package used in the built-in body processors).

It's not a huge issue as you can workaround this by creating your own interface that exposes a Set() method and use that instead, but it's definitely a bit awkward/counter-intuitive.