Add a method to fetch an unhashed key, close #100 (#152)

* Add a method to fetch an unhashed key, close #100

* Return decoded value

* Refactoring

src/lib.rs

@@ -258,6 +258,19 @@ impl<T: Runtime> Client<T> {
         &self.metadata
     }
 
+    /// Fetch the value under an unhashed storage key
+    pub async fn fetch_unhashed<V: Decode>(
+        &self,
+        key: StorageKey,
+        hash: Option<T::Hash>,
+    ) -> Result<Option<V>, Error> {
+        if let Some(data) = self.rpc.storage(&key, hash).await? {
+            Ok(Some(Decode::decode(&mut &data.0[..])?))
+        } else {
+            Ok(None)
+        }
+    }
+
     /// Fetch a StorageKey with an optional block hash.
     pub async fn fetch<F: Store<T>>(
         &self,
@@ -265,11 +278,7 @@ impl<T: Runtime> Client<T> {
         hash: Option<T::Hash>,
     ) -> Result<Option<F::Returns>, Error> {
         let key = store.key(&self.metadata)?;
-        if let Some(data) = self.rpc.storage(&key, hash).await? {
-            Ok(Some(Decode::decode(&mut &data.0[..])?))
-        } else {
-            Ok(None)
-        }
+        self.fetch_unhashed::<F::Returns>(key, hash).await
     }
 
     /// Fetch a StorageKey that has a default value with an optional block hash.