Fixed quoted-printable encoding for Japanese

[t-hashimoto223]

I found two issues.

1. During the quoted-printable encoding process, I found that the characters on the last line were being corrupted.
2. Gmail rejected mail, when generated quoted-printable encoded as multiple line on Subject header.

I tested with the Mailgun and Gmail and all quoted-printable text verified by Emacs in encoding test cases.

[t-hashimoto223]

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[t-hashimoto223]

Fix =??= multiline encoding
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