Add the #unshift method to the middleware stack

The docs suggest that the middleware stack is an Array, so I've added
the unshift method to it. Originally I added some more Array methods,
but it was agreed that they lacked usecases.

actionpack/lib/action_dispatch/middleware/stack.rb

@@ -75,6 +75,11 @@ def [](i)
       middlewares[i]
     end
 
+    def unshift(*args, &block)
+      middleware = self.class::Middleware.new(*args, &block)
+      middlewares.unshift(middleware)
+    end
+
     def initialize_copy(other)
       self.middlewares = other.middlewares.dup
     end

actionpack/test/dispatch/middleware_stack_test.rb

@@ -45,7 +45,7 @@ def setup
     assert_equal BazMiddleware, @stack.last.klass
     assert_equal([true, {:foo => "bar"}], @stack.last.args)
   end
-  
+
   test "use should push middleware class with block arguments onto the stack" do
     proc = Proc.new {}
     assert_difference "@stack.size" do
@@ -54,7 +54,7 @@ def setup
     assert_equal BlockMiddleware, @stack.last.klass
     assert_equal proc, @stack.last.block
   end
-  
+
   test "insert inserts middleware at the integer index" do
     @stack.insert(1, BazMiddleware)
     assert_equal BazMiddleware, @stack[1].klass
@@ -87,6 +87,11 @@ def setup
     assert_equal FooMiddleware, @stack[0].klass
   end
 
+  test "unshift adds a new middleware at the beginning of the stack" do
+    @stack.unshift :"MiddlewareStackTest::BazMiddleware"
+    assert_equal BazMiddleware, @stack.first.klass
+  end
+
   test "raise an error on invalid index" do
     assert_raise RuntimeError do
       @stack.insert("HiyaMiddleware", BazMiddleware)