Regression - replace md5 in Filter module calls with sha256 hashes

Per Drupal secure coding standards at http://drupal.org/node/845876

md5 and sha1 should not be used any place in Drupal core since 7.0, but are re-introduced in the Filter module for 8.x (and also 7.x)

core/modules/filter/filter.module
1715:    $hash = md5($content);

core/modules/filter/lib/Drupal/filter/Tests/FilterHtmlImageSecureTest.php
117:      $comment[] = '<img src="' . $image . '" testattribute="' . md5($image) . '" />';
125:      foreach ($this->xpath('//img[@testattribute="' . md5($image) . '"]') as $element) {