Added tests to validate getContents() works for arbitrary callbacks

Starting with PHP 5.4, you can call arbitrary callbacks using the
`$callback()` notation (`call_user_func()` is no longer necessary).

As such, also reverted the changes from 1b43e4d.

src/CallbackStream.php

@@ -154,8 +154,7 @@ public function read($length)
     public function getContents()
     {
         $callback = $this->detach();
-
-        return $callback ? call_user_fanc($callback) : '';
+        return $callback ? $callback() : '';
     }
 
     /**

test/CallbackStreamTest.php

@@ -18,6 +18,26 @@
  */
 class CallbackStreamTest extends TestCase
 {
+    /**
+     * Sample callback to use with testing.
+     *
+     * @return string
+     */
+    public function sampleCallback()
+    {
+        return __METHOD__;
+    }
+
+    /**
+     * Sample static callback to use with testing.
+     *
+     * @return string
+     */
+    public static function sampleStaticCallback()
+    {
+        return __METHOD__;
+    }
+
     public function testToString()
     {
         $stream = new CallbackStream(function () {
@@ -177,4 +197,26 @@ public function testGetMetadata()
         $notExists = $stream->getMetadata('boo');
         $this->assertNull($notExists);
     }
+
+    public function phpCallbacksForStreams()
+    {
+        $class = 'ZendTest\Diactoros\CallbackStreamTest';
+
+        // @codingStandardsIgnoreStart
+        return [
+            'instance-method' => [[new self(), 'sampleCallback'],   $class . '::sampleCallback'],
+            'static-method'   => [[$class, 'sampleStaticCallback'], $class . '::sampleStaticCallback'],
+        ];
+        // @codingStandardsIgnoreEnd
+    }
+
+    /**
+     * @dataProvider phpCallbacksForStreams
+     */
+    public function testAllowsArbitraryPhpCallbacks($callback, $expected)
+    {
+        $stream = new CallbackStream($callback);
+        $contents = $stream->getContents();
+        $this->assertEquals($expected, $contents);
+    }
 }