mime/quotedprintable: fix encoding where a period alone on a line

Given the following example of message body,

  A line that precisely have length 75 with . + LF will cause DATA truncation.\n
  \n
  Footer.\n

The quotedprintable Writer will encode the message into,

  A line ... truncation=\r\n
  .\r\n
  \r\n
  Footer.\r\n

If we pass the Writer output into SMTP DATA command, the server read
the "\r\n.\r\n" as the end of DATA which cause the message truncated
on the receiver.

This changes fix this issue by encode the period at the end with "=2E"
based on recommendation in RFC 2049 Section 3, point (8) [1].

[1] https://www.rfc-editor.org/rfc/rfc2049.html#section-3

Fixes #61235

Change-Id: I350387b183ac6b25886f4084a060dcfcb48232a9

0 files changed, 0 insertions, 0 deletions