feat(headers): Add Access-Control-Allow-Credentials header

There is only one acceptable value for this header, "true", so
there header struct is unit-like (no fields nor data inside). See
documentation and tests for more info. Case insensitive.

Closes #655

src/header/common/access_control_allow_credentials.rs

@@ -0,0 +1,89 @@
+use std::fmt::{self, Display};
+use std::str;
+use unicase::UniCase;
+use header::{Header, HeaderFormat};
+
+/// `Access-Control-Allow-Credentials` header, part of
+/// [CORS](http://www.w3.org/TR/cors/#access-control-allow-headers-response-header)
+///
+/// > The Access-Control-Allow-Credentials HTTP response header indicates whether the
+/// > response to request can be exposed when the credentials flag is true. When part
+/// > of the response to an preflight request it indicates that the actual request can
+/// > be made with credentials. The Access-Control-Allow-Credentials HTTP header must
+/// > match the following ABNF:
+///
+/// # ABNF
+/// ```plain
+/// Access-Control-Allow-Credentials: "Access-Control-Allow-Credentials" ":" "true"
+/// ```
+///
+/// Since there is only one acceptable field value, the header struct does not accept
+/// any values at all. Setting an empty `AccessControlAllowCredentials` header is
+/// sufficient. See the examples below.
+///
+/// # Example values
+/// * "true"
+///
+/// # Examples
+/// ```
+/// # extern crate hyper;
+/// # fn main() {
+///
+/// use hyper::header::{Headers, AccessControlAllowCredentials};
+///
+/// let mut headers = Headers::new();
+/// headers.set(AccessControlAllowCredentials);
+/// # }
+/// ```
+#[derive(Clone, PartialEq, Debug)]
+pub struct AccessControlAllowCredentials;
+
+const ACCESS_CONTROL_ALLOW_CREDENTIALS_TRUE: UniCase<&'static str> = UniCase("true");
+
+impl Header for AccessControlAllowCredentials {
+    fn header_name() -> &'static str {
+        "Access-Control-Allow-Credentials"
+    }
+
+    fn parse_header(raw: &[Vec<u8>]) -> ::Result<AccessControlAllowCredentials> {
+        if raw.len() == 1 {
+            let text = unsafe {
+                // safe because:
+                // 1. we just checked raw.len == 1
+                // 2. we don't actually care if it's utf8, we just want to
+                //    compare the bytes with the "case" normalized. If it's not
+                //    utf8, then the byte comparison will fail, and we'll return
+                //    None. No big deal.
+                str::from_utf8_unchecked(raw.get_unchecked(0))
+            };
+            if UniCase(text) == ACCESS_CONTROL_ALLOW_CREDENTIALS_TRUE {
+                return Ok(AccessControlAllowCredentials);
+            }
+        }
+        Err(::Error::Header)
+    }
+}
+
+impl HeaderFormat for AccessControlAllowCredentials {
+    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("true")
+    }
+}
+
+impl Display for AccessControlAllowCredentials {
+    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+        self.fmt_header(f)
+    }
+}
+
+#[cfg(test)]
+mod test_access_control_allow_credentials {
+    use std::str;
+    use header::*;
+    use super::AccessControlAllowCredentials as HeaderField;
+    test_header!(works,        vec![b"true"], Some(HeaderField));
+    test_header!(ignores_case, vec![b"True"]);
+    test_header!(not_bool,     vec![b"false"], None);
+    test_header!(only_single,  vec![b"true", b"true"], None);
+    test_header!(no_gibberish, vec!["\u{645}\u{631}\u{62d}\u{628}\u{627}".as_bytes()], None);
+}

src/header/common/mod.rs

@@ -7,6 +7,7 @@
 //! is used, such as `ContentType(pub Mime)`.
 
 pub use self::accept::Accept;
+pub use self::access_control_allow_credentials::AccessControlAllowCredentials;
 pub use self::access_control_allow_headers::AccessControlAllowHeaders;
 pub use self::access_control_allow_methods::AccessControlAllowMethods;
 pub use self::access_control_allow_origin::AccessControlAllowOrigin;
@@ -353,6 +354,7 @@ macro_rules! header {
 
 
 mod accept;
+mod access_control_allow_credentials;
 mod access_control_allow_headers;
 mod access_control_allow_methods;
 mod access_control_allow_origin;