Use &self instead of &mut self for ReadHalf::peek

[al8n]

Hi, this PR remove the change &mut self to &self in ReadHalf::peek.

[Darksonn]

This is a breaking change in the main Tokio crate. It's not going to happen.

[al8n]

This is a breaking change in the main Tokio crate. It's not going to happen.

But it is weird, because TcpStream::peek requires &self, but ReadHalf require &mut self

[al8n]

Here are the documents:

1. TcpStream::peek: https://docs.rs/tokio/latest/tokio/net/struct.TcpStream.html#method.peek
2. ReadHalf::peek: https://docs.rs/tokio/latest/tokio/net/tcp/struct.ReadHalf.html#method.peek
3. OwnedReadHalf::peek: https://docs.rs/tokio/latest/tokio/net/tcp/struct.OwnedReadHalf.html#method.peek

The ReadHalf::peek and OwnedReadHalf::peek signatures differ from TcpStream::peek.

[Darksonn]

We could fix that, but we will not because it's a breaking change. You can always call TcpStream::peek via .as_ref() since that lets you access the underlying IO resource from a read half.