Fix for PHP7 & Mysql 5.7

- added default values to not null columns
- fixed passing by reference issue on MailChimp->getDataCenter
- fixed a couple unit tests

lib/Subscribers/ImportExport/Import/MailChimp.php

@@ -121,7 +121,8 @@ class MailChimp {
   function getDataCenter($APIKey) {
     if (!preg_match('/-[a-zA-Z0-9]{3,}/', $APIKey)) return false;
     // double parantheses: http://phpsadness.com/sad/51
-    return end((explode('-', $APIKey)));
+    $key_parts = explode('-', $APIKey);
+    return end($key_parts);
   }
 
   function getAPIKey($APIKey) {