feat(body): remove Body::is_empty()

BREAKING CHANGE: `Body::is_empty()` is gone. Replace with
  `Body::is_end_stream()`, from the `Payload` trait.

src/body.rs

@@ -224,24 +224,6 @@ impl Body {
         Body::new(Kind::Wrapped(Box::new(mapped)))
     }
 
-    /// Returns if this body was constructed via `Body::empty()`.
-    ///
-    /// # Note
-    ///
-    /// This does **not** detect if the body stream may be at the end, or
-    /// if the stream will not yield any chunks, in all cases. For instance,
-    /// a streaming body using `chunked` encoding is not able to tell if
-    /// there are more chunks immediately.
-    ///
-    /// See [`is_end_stream`](Payload::is_end_stream) for a dynamic version.
-    #[inline]
-    pub fn is_empty(&self) -> bool {
-        match self.kind {
-            Kind::Empty => true,
-            _ => false,
-        }
-    }
-
     fn new(kind: Kind) -> Body {
         Body {
             kind: kind,

src/client/mod.rs

@@ -370,7 +370,7 @@ where C: Connect + Sync + 'static,
                         // It won't be ready if there is a body to stream.
                         if ver == Ver::Http2 || !pooled.is_pool_enabled() || pooled.is_ready() {
                             drop(pooled);
-                        } else if !res.body().is_empty() {
+                        } else if !res.body().is_end_stream() {
                             let (delayed_tx, delayed_rx) = oneshot::channel();
                             res.body_mut().delayed_eof(delayed_rx);
                             executor.execute(