minor symfony#24374 [HttpFoundation] Return instance in StreamedResponse (abellion)

This PR was squashed before being merged into the 3.4 branch (closes symfony#24374).

Discussion
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[HttpFoundation] Return instance in StreamedResponse

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

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Commits
-------

2080582 [HttpFoundation] Return instance in StreamedResponse

Author: fabpot

src/Symfony/Component/HttpFoundation/StreamedResponse.php

@@ -66,16 +66,22 @@ public static function create($callback = null, $status = 200, $headers = array(
      * Sets the PHP callback associated with this Response.
      *
      * @param callable $callback A valid PHP callback
+     *
+     * @return $this
      */
     public function setCallback(callable $callback)
     {
         $this->callback = $callback;
+
+        return $this;
     }
 
     /**
      * {@inheritdoc}
      *
      * This method only sends the headers once.
+     *
+     * @return $this
      */
     public function sendHeaders()
     {
@@ -85,13 +91,15 @@ public function sendHeaders()
 
         $this->headersSent = true;
 
-        parent::sendHeaders();
+        return parent::sendHeaders();
     }
 
     /**
      * {@inheritdoc}
      *
      * This method only sends the content once.
+     *
+     * @return $this
      */
     public function sendContent()
     {
@@ -106,18 +114,24 @@ public function sendContent()
         }
 
         call_user_func($this->callback);
+
+        return $this;
     }
 
     /**
      * {@inheritdoc}
      *
      * @throws \LogicException when the content is not null
+     *
+     * @return $this
      */
     public function setContent($content)
     {
         if (null !== $content) {
             throw new \LogicException('The content cannot be set on a StreamedResponse instance.');
         }
+
+        return $this;
     }
 
     /**