Add 'Bounced' status to subscribers [MAILPOET-684]

Fix SubscriberTest->testItHasGroupFilter() making only one of four assertions due to lack of data.
Fix a small issue with Import. Unexpected values for the status field were converted to 'unconfirmed' rather than 'subscribed' due to non-strict comparison in in_array(). This hindered 'bounced' status import as well.

lib/Subscription/Pages.php

@@ -290,6 +290,14 @@ class Pages {
               'is_checked' => (
                 $subscriber->status === Subscriber::STATUS_UNSUBSCRIBED
               )
+            ),
+            array(
+              'value' => array(
+                Subscriber::STATUS_BOUNCED => __('Bounced', 'mailpoet')
+              ),
+              'is_checked' => (
+                $subscriber->status === Subscriber::STATUS_BOUNCED
+              )
             )
           )
         )