fix(http1): only send 100 Continue if request body is polled

Before, if a client request included an `Expect: 100-continue` header,
the `100 Continue` response was sent immediately. However, this is
problematic if the service is going to reply with some 4xx status code
and reject the body.

This change delays the automatic sending of the `100 Continue` status
until the service has call `poll_data` on the request body once.

src/proto/h1/mod.rs

@@ -74,3 +74,22 @@ pub(crate) struct Encode<'a, T> {
     req_method: &'a mut Option<Method>,
     title_case_headers: bool,
 }
+
+/// Extra flags that a request "wants", like expect-continue or upgrades.
+#[derive(Clone, Copy, Debug)]
+struct Wants(u8);
+
+impl Wants {
+    const EMPTY: Wants = Wants(0b00);
+    const EXPECT: Wants = Wants(0b01);
+    const UPGRADE: Wants = Wants(0b10);
+
+    #[must_use]
+    fn add(self, other: Wants) -> Wants {
+        Wants(self.0 | other.0)
+    }
+
+    fn contains(&self, other: Wants) -> bool {
+        (self.0 & other.0) == other.0
+    }
+}