minor symfony#54504 [HttpClient] fix syntax for PHP 7.2 (xabbuh)

This PR was merged into the 5.4 branch.

Discussion
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[HttpClient] fix syntax for PHP 7.2

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

Commits
-------

d5094eb fix syntax for PHP 7.2

Author: xabbuh

src/Symfony/Component/HttpClient/Tests/EventSourceHttpClientTest.php

@@ -34,7 +34,7 @@ class EventSourceHttpClientTest extends TestCase
      */
     public function testGetServerSentEvents(string $sep)
     {
-        $data = str_replace("\n", $sep, <<<TXT
+        $rawData = <<<TXT
 event: builderror
 id: 46
 data: {"foo": "bar"}
@@ -62,7 +62,8 @@ public function testGetServerSentEvents(string $sep)
 
 id: 60
 data
-TXT);
+TXT;
+        $data = str_replace("\n", $sep, $rawData);
 
         $chunk = new DataChunk(0, $data);
         $response = new MockResponse('', ['canceled' => false, 'http_method' => 'GET', 'url' => 'http://localhost:8080/events', 'response_headers' => ['content-type: text/event-stream']]);