[HttpClient] always return the empty string when the response cannot have a body

Author: nicolas-grekas

Response/ResponseTrait.php

@@ -112,11 +112,15 @@ public function getContent(bool $throw = true): string
                 }
             }
 
-            if (null === $content) {
-                throw new TransportException('Cannot get the content of the response twice: the request was issued with option "buffer" set to false.');
+            if (null !== $content) {
+                return $content;
             }
 
-            return $content;
+            if ('HEAD' === $this->info['http_method'] || \in_array($this->info['http_code'], [204, 304], true)) {
+                return '';
+            }
+
+            throw new TransportException('Cannot get the content of the response twice: the request was issued with option "buffer" set to false.');
         }
 
         foreach (self::stream([$this]) as $chunk) {

Tests/MockHttpClientTest.php

@@ -48,7 +48,7 @@ protected function getHttpClient(string $testCase): HttpClientInterface
                 return new MockHttpClient(function (string $method, string $url, array $options) use ($client) {
                     try {
                         // force the request to be completed so that we don't test side effects of the transport
-                        $response = $client->request($method, $url, $options);
+                        $response = $client->request($method, $url, ['buffer' => false] + $options);
                         $content = $response->getContent(false);
 
                         return new MockResponse($content, $response->getInfo());