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