feat(header): introduce header::Raw (#869)

The Raw type repesents the raw bytes of a header-value.

Having a special type allows a couple of benefits:

- The exact representation has become private, allowing "uglier"
internals. Specifically, since the common case is for a header to only
have 1 line of bytes, an enum is used to skip allocating a Vec for only
1 line. Additionally, a Cow<'static, [u8]> is used, so static bytes
don't require a copy. Finally, since we can use static bytes, when
parsing, we can compare the incoming bytes against a couple of the most
common header-values, and possibly remove another copy.

- As its own type, the `Headers.set_raw` method can be generic over
`Into<Raw>`, which allows for more ergnomic method calls.

BREAKING CHANGE: `Header::parse_header` now receives `&Raw`, instead of
  a `&[Vec<u8>]`. `Raw` provides several methods to ease using it, but
  may require some changes to existing code.

src/header/parsing.rs

@@ -6,13 +6,17 @@ use std::str::FromStr;
 use std::fmt::{self, Display};
 use url::percent_encoding;
 
+use header::Raw;
 use header::shared::Charset;
 
 /// Reads a single raw string when parsing a header.
-pub fn from_one_raw_str<T: str::FromStr>(raw: &[Vec<u8>]) -> ::Result<T> {
-    if raw.len() != 1 || unsafe { raw.get_unchecked(0) } == b"" { return Err(::Error::Header) }
-    // we JUST checked that raw.len() == 1, so raw[0] WILL exist.
-    from_raw_str( unsafe { raw.get_unchecked(0) })
+pub fn from_one_raw_str<T: str::FromStr>(raw: &Raw) -> ::Result<T> {
+    if let Some(line) = raw.one() {
+        if !line.is_empty() {
+            return from_raw_str(line)
+        }
+    }
+    Err(::Error::Header)
 }
 
 /// Reads a raw string into a value.
@@ -23,7 +27,7 @@ pub fn from_raw_str<T: str::FromStr>(raw: &[u8]) -> ::Result<T> {
 
 /// Reads a comma-delimited raw header into a Vec.
 #[inline]
-pub fn from_comma_delimited<T: str::FromStr, S: AsRef<[u8]>>(raw: &[S]) -> ::Result<Vec<T>> {
+pub fn from_comma_delimited<T: str::FromStr>(raw: &Raw) -> ::Result<Vec<T>> {
     let mut result = Vec::new();
     for s in raw {
         let s = try!(str::from_utf8(s.as_ref()));