io: delegate poll_write_vectored to inner

[M0dEx]

Motivation

The current implementation of AsyncWrite for WriteHalf<T> does not delegate its implementation of poll_write_vectored and is_write_vectored to the underlying struct. Instead, the default implementation is called, leading to unexpected behaviour.

Solution

This PR delegates the implementation poll_write_vectored and is_write_vectored to the underlying struct, fixing the issue.
It also adds a test to verify the delegation works as intended.

Fixes #5913.

TODO

• delegate implementation of poll_write_vectored
• delegate implementation of is_write_vectored (any tips on how to achieve this with the current method signature would be appreciated)

[Darksonn]

Thanks.