Add utf-8 preamble to admin_scripts temp settings file

As some settings are copied from current user settings, we cannot
be sure the characters are pure ascii.

tests/admin_scripts/tests.py

@@ -45,6 +45,7 @@ class AdminScriptTestCase(unittest.TestCase):
             settings_file_path = os.path.join(test_dir, filename)
 
         with open(settings_file_path, 'w') as settings_file:
+            settings_file.write('# -*- coding: utf-8 -*\n')
             settings_file.write('# Settings file automatically generated by admin_scripts test case\n')
             exports = [
                 'DATABASES',