Fix #2499

The mail domain can now be configured in config.php and get´s used in function getDefaultEmailAddress.

e.g. 'mail_domain' => 'example.com'
Extend test case for getDefaultEmailAddress()

see #2499
Move testcase to own test function

Move testcase for getDefaultEmailAddress with mail_domain set in config.php to own test function.
Update to reflect new parameter mail_domain

see #2568
fix missing ',' after "mail_domain" parameter
Merge pull request #2568 from itheiss/master

Implement setting custom domainname for getDefaultEmailAddress