normalizing emails in lowercase

lib/Subscribers/ImportExport/Import/Import.php

@@ -159,7 +159,7 @@ class Import {
             if(!is_email($email)) {
               $invalid_records[] = $index;
             }
-            return $email;
+            return strtolower($email);
           }, array_keys($data), $data
         );
       }

tests/unit/Subscribers/ImportExport/Import/ImportTest.php

@@ -23,7 +23,7 @@ class ImportTest extends \MailPoetTest {
         array(
           'Adam',
           'Smith',
-          'adam@smith.com',
+          'Adam@smith.com',
           'France'
         ),
         array(