Issue:
`custom_headers` is available as an optional keyword argument in `build_mime_message` but it is not exposed in `send_email` or `send_email_smtp`. This prevents using custom headers with the email operator when using the default smtp backend. 

https://github.com/apache/airflow/blob/8e2a0bc2e39aeaf15b409bbfa8ac0c85aa873815/airflow/utils/email.py#L105-L116

Expected:
`send_email` should take optional key word argument `custom_headers` and pass it to `send_email_smtp` if that is the selected backend. 




tests/operators/test_email.py
#
#
#
import datetime
import unittest
from unittest import mock
from airflow.models.dag import DAG
from airflow.operators.email import EmailOperator
from airflow.utils import timezone
from tests.test_utils.config import conf_vars
DEFAULT_DATE = timezone.datetime(2016, 1, 1)
END_DATE = timezone.datetime(2016, 1, 2)
INTERVAL = datetime.timedelta(hours=12)
FROZEN_NOW = timezone.datetime(2016, 1, 2, 12, 1, 1)
send_email_test = mock.Mock()
class TestEmailOperator(unittest.TestCase):