fix(headers): Do not parse empty values in list headers.

In empty list header values ``, or list header values with empty items `foo, , bar`,
the empty value is parsed, if the parser does not reject empty values an item is added
to the resulting header. There can't be empty values. Added a test for it in AcceptEncoding.

src/header/common/accept_encoding.rs

@@ -20,7 +20,7 @@ header! {
     test_accept_encoding {
         // From the RFC
         test_header!(test1, vec![b"compress, gzip"]);
-        test_header!(test2, vec![b""]);
+        test_header!(test2, vec![b""], Some(AcceptEncoding(vec![])));
         test_header!(test3, vec![b"*"]);
         // Note: Removed quality 1 from gzip
         test_header!(test4, vec![b"compress;q=0.5, gzip"]);