[AIRFLOW-2765] Set default mime_charset to UTF-8
[AIRFLOW-2765] Set default mime_charset to UTF-8

Closes #3627 from jeffkpayne/AIRFLOW-2765

lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow