Clean up RequestParts API (#167)

In http-body 0.4.3 `BoxBody` implements `Default`. This allows us to
clean up the API of `RequestParts` quite a bit.

src/error.rs

@@ -13,6 +13,16 @@ impl Error {
             inner: error.into(),
         }
     }
+
+    pub(crate) fn downcast<T>(self) -> Result<T, Self>
+    where
+        T: StdError + 'static,
+    {
+        match self.inner.downcast::<T>() {
+            Ok(t) => Ok(*t),
+            Err(err) => Err(*err.downcast().unwrap()),
+        }
+    }
 }
 
 impl fmt::Display for Error {