Fix ActionMailer testcase break with mail 2.5.4.

According to https://travis-ci.org/rails/rails/jobs/7220308#L148, AM's testcase was break.
I guess that AM's testcase was wrong. When we pass :transfer_encoding option, we should pass *encoded* content.

I guess this PR should be backported to 3-2-stable, 4-0-stable too.