Split common headers into a submodule and into their own files

This is a more extensible place to put them and doesn't clutter up
header/mod.rs as much as the old scheme did.

Fixes #8

src/header/common/mod.rs

@@ -0,0 +1,59 @@
+//! A Collection of Header implementations for common HTTP Headers.
+//!
+//! ## Mime
+//!
+//! Several header fields use MIME values for their contents. Keeping with the
+//! strongly-typed theme, the [mime](http://seanmonstar.github.io/mime.rs) crate
+//! is used, such as `ContentType(pub Mime)`.
+
+pub use self::host::Host;
+pub use self::content_length::ContentLength;
+pub use self::content_type::ContentType;
+pub use self::accept::Accept;
+pub use self::connection::Connection;
+pub use self::transfer_encoding::TransferEncoding;
+pub use self::user_agent::UserAgent;
+pub use self::server::Server;
+pub use self::date::Date;
+
+use std::from_str::FromStr;
+use std::str::from_utf8;
+
+/// Exposes the Host header.
+pub mod host;
+
+/// Exposes the ContentLength header.
+pub mod content_length;
+
+/// Exposes the ContentType header.
+pub mod content_type;
+
+/// Exposes the Accept header.
+pub mod accept;
+
+/// Exposes the Connection header.
+pub mod connection;
+
+/// Exposes the TransferEncoding header.
+pub mod transfer_encoding;
+
+/// Exposes the UserAgent header.
+pub mod user_agent;
+
+/// Exposes the Server header.
+pub mod server;
+
+/// Exposes the Date header.
+pub mod date;
+
+fn from_one_raw_str<T: FromStr>(raw: &[Vec<u8>]) -> Option<T> {
+    if raw.len() != 1 {
+        return None;
+    }
+    // we JUST checked that raw.len() == 1, so raw[0] WILL exist.
+    match from_utf8(unsafe { raw.as_slice().unsafe_get(0).as_slice() }) {
+        Some(s) => FromStr::from_str(s),
+        None => None
+    }
+}
+