Avoid accidental overwrites of original configuration

[jonathanjfshaw]

From #182:

+    $config = \Drupal::configFactory()->getEditable('system.mail');
+    $data = $config->getRawData();
+
+    // Save the original values for restoration after.
+    $this->originalConfiguration['system.mail'] = $data;

If startCollectingMail is called twice before stopCollectingMail is called (and I think this might happen in the parrallel behat extension PR), then test_mail_collector will end up being stored in $originalConfiguration and persisted.

Therefore we should not overwrite values in originalConfiguration.