feat(http/body-eos): EosRef<'a, E>: Clone + Copy

this commit introduces clone and copy implementations for the
`EosRef<'a, E>` type used to inspect and classify the outcome of a
request or response body.

as the documentation comment notes in the commit below, this enum only
contains immutable references to other values: a header map or an
error. other variants are empty tags.

```rust
pub enum EosRef<'a, E = Error> {
    None,
    Trailers(&'a HeaderMap),
    Error(&'a E),
    Cancelled,
}
```

this commit makes this type implicitly clonable.

Signed-off-by: katelyn martin <kate@buoyant.io>

Author: cratelyn

linkerd/http/body-eos/src/lib.rs

@@ -130,3 +130,16 @@ where
         };
     }
 }
+
+// === impl EosRef ===
+
+/// A reference to the end of a stream can be cheaply cloned.
+///
+/// Each of the variants are empty tags, or immutable references.
+impl<'a, E> Clone for EosRef<'a, E> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<'a, E> Copy for EosRef<'a, E> {}