Header::parse_header() returns now a hyper Result instead of an option
this will enable more precise Error messages in the future, currently
most failures are reported as ::Error::Header.
BREAKING CHANGE: parse_header returns Result instead of Option, related
code did also change
BREAKING CHANGE: added requirement that all HeaderFormat implementations
must also be fmt::Debug. This likely as easy as slapping
#[derive(Debug)] on to any custom headers.