Remove deprecated get_mut_ref usage.
Remove deprecated get_mut_ref usage.

servo/rust-http,servo/rust-http,Shraddha512/rust-http,Shraddha512/rust-http,chris-morgan/rust-http,chris-morgan/rust-http,servo/rust-http,Shraddha512/rust-http,servo/rust-http,chris-morgan/rust-http,Shraddha512/rust-http,chris-morgan/rust-http,Shraddha512/rust-http