Broken `test_bonds` test function in `maps.rs`.

[Sol-Ell]

Its a minor issue, but currently the trait bounds of the T in check_bounds are Send and Send.

https://github.com/hyperium/http/blob/9f86d52c1310c485a3128f3c86c0ca7277fe5d92/src/header/map.rs#L3914-L3918

I think the author of that test meant fn check_bounds<T: Send + Sync> {}.