buffer: remove minor Buffer::Copy deoptimizations
buffer: remove minor Buffer::Copy deoptimizations

* Omit ToObject() call. Buffer::Data() and Buffer::Length() know how
  to deal with Values.

* Don't check if the argument is undefined because it realistically
  never is and undefined->integer coercion achieves the same thing.

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node