feat(client): Response.status() now returns a StatusCode

Previously, it would return `&StatusCode`. Returning a reference was
actually bigger than the enum itself, and prevented using `Into` on the
return result directly.

BREAKING CHANGE: If you were explicitly checking the status, such as
  with an equality comparison, you will need to use the value instead of a
  reference.

src/status.rs

@@ -550,9 +550,9 @@ impl Default for StatusCode {
     }
 }
 
-impl Into<u16> for StatusCode {
-    fn into(self) -> u16 {
-        self.to_u16()
+impl From<StatusCode> for u16 {
+    fn from(code: StatusCode) -> u16 {
+        code.to_u16()
     }
 }