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/expect.rs

@@ -0,0 +1,37 @@
+use std::fmt;
+
+use header::{Header, HeaderFormat};
+
+/// The `Expect` header.
+///
+/// > The "Expect" header field in a request indicates a certain set of
+/// > behaviors (expectations) that need to be supported by the server in
+/// > order to properly handle this request.  The only such expectation
+/// > defined by this specification is 100-continue.
+/// >
+/// >    Expect  = "100-continue"
+#[derive(Copy, Clone, PartialEq, Debug)]
+pub enum Expect {
+    /// The value `100-continue`.
+    Continue
+}
+
+impl Header for Expect {
+    fn header_name() -> &'static str {
+        "Expect"
+    }
+
+    fn parse_header(raw: &[Vec<u8>]) -> Option<Expect> {
+        if &[b"100-continue"] == raw {
+            Some(Expect::Continue)
+        } else {
+            None
+        }
+    }
+}
+
+impl HeaderFormat for Expect {
+    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("100-continue")
+    }
+}