Fix Symfony 3.2 deprecation notices

Since May 4, https://www.drupal.org/pift-ci-job/689665 is failing:

fail: [Other] Line 0 of sites/default/files/simpletest/phpunit-2.xml:
PHPunit Test failed to complete; Error: PHPUnit 4.8.35 by Sebastian Bergmann and contributors.

Testing Drupal\Tests\big_pipe_sessionless\Unit\Render\Placeholder\BigPipeSessionlessStrategyTest
..........

Time: 178 ms, Memory: 6.00MB

OK (10 tests, 24 assertions)

Remaining deprecation notices (20)

Checking only for cacheable HTTP methods with Symfony\Component\HttpFoundation\Request::isMethodSafe() is deprecated since version 3.2 and will throw an exception in 4.0. Disable checking only for cacheable methods by calling the method with `false` as first argument or use the Request::isMethodCacheable() instead: 20x
    20x in BigPipeSessionlessStrategyTest::testProcessPlaceholders from Drupal\Tests\big_pipe_sessionless\Unit\Render\Placeholder

The solution is simple: replace isMethodSafe() calls with isMethodCacheable() calls, just like #2712647: Update Symfony components to ~3.2 did.