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
A single value header value can't be "", so `from_one_raw_str()` now
returns `None` on empty values. This makes custom checks in headers
obsolete.
BREAKING CHANGE: `from_one_raw_str()` returns `None` on empty values.
Add the HTTP/1.0 `Pragma` header field used to prevent older Caches, that
do not understand the `Cache-Control` header field from caching the ressource.
Closes#237