buffer: return this in fill() for chainability

[mscdex]

The parent fill() just returns undefined, so return something more useful for chaining purposes instead.

[trevnorris]

The return value should come from Buffer::Fill. Just return args.This(). I'm on the fence about the change.

/cc @isaacs

[mscdex]

Wouldn't that return a reference to the parent and not the slice?

[trevnorris]

ah yeah, you're right. but that will change once #4964 lands.

[bnoordhuis]

Landed in 6af8788, thanks.