Added default value for middleware

Sources/SwiftDux/Store/Store.swift

@@ -21,7 +21,7 @@ public final class Store<State> where State : StateType {
   ///   - state: The initial state of the store. A typically use case is to restore a previous application session with a persisted state object.
   ///   - reducer: A reducer that will mutate the store's state as actions are dispatched to it.
   ///   - middleware: One or more middleware plugins
-  public init<R>(state: State, reducer: R, middleware: [Middleware<State>]) where R : Reducer, R.State == State {
+  public init<R>(state: State, reducer: R, middleware: [Middleware<State>] = []) where R : Reducer, R.State == State {
     let storeReducer = StoreReducer(reducer)
     self.state = state
     self.reduceAction = middleware.reversed().reduce(