Compose an EmitterStack by default

- It makes sense to compose an EmitterStack by default; that way, users
  of AppFactory can manipulate the emitter without needing to resort to
  direct instantiation.

Author: weierophinney

src/Application.php

@@ -458,15 +458,16 @@ public function getFinalHandler(ResponseInterface $response = null)
     /**
      * Retrieve an emitter to use during run().
      *
-     * If none was registered during instantiation, this will lazy-load a
-     * SapiEmitter instance.
+     * If none was registered during instantiation, this will lazy-load an
+     * EmitterStack composing an SapiEmitter instance.
      *
      * @return EmitterInterface
      */
     public function getEmitter()
     {
         if (! $this->emitter) {
-            $this->emitter = new SapiEmitter;
+            $this->emitter = new Emitter\EmitterStack();
+            $this->emitter->push(new SapiEmitter());
         }
         return $this->emitter;
     }

test/ApplicationTest.php

@@ -13,8 +13,10 @@
 use PHPUnit_Framework_TestCase as TestCase;
 use Prophecy\Argument;
 use ReflectionProperty;
+use Zend\Diactoros\Response\SapiEmitter;
 use Zend\Diactoros\ServerRequest as Request;
 use Zend\Expressive\Application;
+use Zend\Expressive\Emitter\EmitterStack;
 use Zend\Expressive\Router\Route;
 use Zend\Expressive\Router\RouteResult;
 use Zend\Stratigility\Route as StratigilityRoute;
@@ -260,11 +262,15 @@ public function testFinalHandlerIsUsedAtInvocationIfNoOutArgumentIsSupplied()
         $this->assertSame($finalResponse, $test);
     }
 
-    public function testComposesSapiEmitterByDefault()
+    public function testComposesEmitterStackWithSapiEmitterByDefault()
     {
-        $app     = $this->getApp();
-        $emitter = $app->getEmitter();
-        $this->assertInstanceOf('Zend\Diactoros\Response\SapiEmitter', $emitter);
+        $app   = $this->getApp();
+        $stack = $app->getEmitter();
+        $this->assertInstanceOf(EmitterStack::class, $stack);
+
+        $this->assertCount(1, $stack);
+        $test = $stack->pop();
+        $this->assertInstanceOf(SapiEmitter::class, $test);
     }
 
     public function testAllowsInjectingEmitterAtInstantiation()