Make $amp_default_role case insensitive

[adelphi-company]

Took me a while to figure out why it did not work at my site, so might be useful to others:

Works:
$amp_default_role = "Editor";

Doesn't work (Error: "Require permissions to show admin interface."):
$amp_default_role = "editor";

Even if it's lower case in:

$amp_role_assignment = array(
  'administrator' => array(
    'your_username',
  ),
  '**editor**' => array(
    'your_close_friend',
  ),
  'contributor' => array(
    'your_other_friend',
  ),
);