fn canonical_values(values: &[Vec<u8>]) -> String {
    let mut st = String::new();
    for v in values {
        let s = str::from_utf8(v).unwrap();
        if !st.is_empty() {
            st.push(',')
        }
        if s.starts_with('\"') {
            st.push_str(s);
        } else {
            st.push_str(s.replace("  ", " ").trim());
        }
    }
    st
}
Canonicalizes values into the AWS Canonical Form.

Read more about it: [HERE](http://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html)
rusoto/signature/src/signature.rs