Content-MD5 header support for hyperium/headers

This adds the RFC1864 Content-MD5 header as a typed header:

```rust use headers::Header; use http::HeaderValue; use headerscontentmd5::ContentMd5;

fn itworks() { let value = HeaderValue::fromstatic("Q2hlY2sgSW50ZWdyaXR5IQ=="); let md5 = ContentMd5::decode(&mut [&value].into_iter()).unwrap();

let expected = "Check Integrity!".as_bytes().try_into().unwrap();
assert_eq!(md5, ContentMd5(expected))

} ```