refactor(headers): Fail to parse single value header values

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.

src/header/common/pragma.rs

@@ -57,4 +57,6 @@ fn test_parse_header() {
     let c: Pragma = Header::parse_header([b"FoObar".to_vec()].as_ref()).unwrap();
     let d = Pragma::Ext("FoObar".to_string());
     assert_eq!(c, d);
+    let e: Option<Pragma> = Header::parse_header([b"".to_vec()].as_ref());
+    assert_eq!(e, None);
 }