non snake case argument

fixes this warning on the build

    src/huffman_coding.rs:96:23: 96:31 warning: variable `startStr` should have a snake case name such
    as `start_str`, #[warn(non_snake_case)] on by default
    src/huffman_coding.rs:96                     startStr: &str) {
                                                 ^~~~~~~~
Maybe it would be more clear to use `format!("{}0", start_str).as_slice()`. Just bikeshedding... What do you think about it?