Issue #7472: ISO-2022 charsets now consistently use 7bit CTE.

Issue #7472: ISO-2022 charsets now consistently use 7bit CTE.

Fixed a typo in the email.encoders module so that messages output using
an ISO-2022 character set will use a content-transfer-encoding of
7bit consistently.  Previously if the input data had any eight bit
characters the output data would get marked as 8bit even though it
was actually 7bit.