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);
 }

src/header/parsing.rs

@@ -9,10 +9,12 @@ pub fn from_one_raw_str<T: str::FromStr>(raw: &[Vec<u8>]) -> Option<T> {
         return None;
     }
     // we JUST checked that raw.len() == 1, so raw[0] WILL exist.
-    match str::from_utf8(&raw[0][..]) {
-        Ok(s) => str::FromStr::from_str(s).ok(),
-        Err(_) => None
+    if let Ok(s) = str::from_utf8(&raw[0][..]) {
+        if s != "" {
+            return str::FromStr::from_str(s).ok();
         }
+    }
+    None
 }
 
 /// Reads a comma-delimited raw header into a Vec.