feat(server): add Expect 100-continue support

Adds a new method to `Handler`, with a default implementation of always
responding with a `100 Continue` when sent an expectation.

Closes #369

src/header/common/mod.rs

@@ -20,6 +20,7 @@ pub use self::content_type::ContentType;
 pub use self::cookie::Cookie;
 pub use self::date::Date;
 pub use self::etag::Etag;
+pub use self::expect::Expect;
 pub use self::expires::Expires;
 pub use self::host::Host;
 pub use self::if_match::IfMatch;
@@ -160,6 +161,7 @@ mod content_length;
 mod content_type;
 mod date;
 mod etag;
+mod expect;
 mod expires;
 mod host;
 mod if_match;