rust: fix clippy warning manual_contains

rust: fix clippy warning manual_contains

warning: using `contains()` instead of `iter().any()` is more efficient
   --> src/http2/http2.rs:267:20
    |
267 |                 if block.value.iter().any(|&x| x == b'@') {
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `block.value.contains(&b'@')`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_contains
    = note: `#[warn(clippy::manual_contains)]` on by default