Merge branch '6.3' into 6.4

* 6.3:
  Add missing return type
  [HttpClient] Fix pausing responses before they start when using curl
  separate child and parent context in NotificationEmail on writes
  [Mailer] [Mailgun] Fix sender header encoding
  do not overwrite the cache key when it is false
  [Mailer] Throw TransportException when unable to read from socket
  [Serializer] Rewrite `AbstractObjectNormalizer::createChildContext()` to use the provided `cache_key` from original context when creating child contexts
  [HttpClient] Fix error chunk creation in passthru
  Adjusting and removing the 'review' attribute from the pt_br translation XML.
  [Serializer] Take unnamed variadic parameters into account when denormalizing
  Fix context data and display extra data

Author: nicolas-grekas

Transport/Smtp/Stream/AbstractStream.php

@@ -77,7 +77,7 @@ public function readLine(): string
             return '';
         }
 
-        $line = fgets($this->out);
+        $line = @fgets($this->out);
         if ('' === $line || false === $line) {
             $metas = stream_get_meta_data($this->out);
             if ($metas['timed_out']) {
@@ -86,6 +86,9 @@ public function readLine(): string
             if ($metas['eof']) {
                 throw new TransportException(sprintf('Connection to "%s" has been closed unexpectedly.', $this->getReadConnectionDescription()));
             }
+            if (false === $line) {
+                throw new TransportException(sprintf('Unable to read from connection to "%s": ', $this->getReadConnectionDescription()).error_get_last()['message']);
+            }
         }
 
         $this->debug .= sprintf('< %s', $line);