Merge branch '3.1'

* 3.1:
  prefer getSourceContext() over getSource()
  [HttpFoundation] Avoid implicit null to array conversion in request matcher
  Revert "bug #20184 [FrameworkBundle] Convert null prefix to an empty string in translation:update (chalasr)"
  Update UPGRADE-2.7.md

Author: fabpot

Command/TranslationUpdateCommand.php

@@ -132,9 +132,8 @@ protected function execute(InputInterface $input, OutputInterface $output)
         // load any messages from templates
         $extractedCatalogue = new MessageCatalogue($input->getArgument('locale'));
         $io->comment('Parsing templates...');
-        $prefix = $input->getOption('prefix');
         $extractor = $this->getContainer()->get('translation.extractor');
-        $extractor->setPrefix(null === $prefix ? '' : $prefix);
+        $extractor->setPrefix($input->getOption('prefix'));
         foreach ($transPaths as $path) {
             $path .= 'views';
             if (is_dir($path)) {