You have to initialize Mail_Mime
correctly:
$mime = new \Mail_mime(
array(
'eol' => "\n",
'head_charset' => 'utf-8',
'text_charset' => 'utf-8',
'html_charset' => 'utf-8',
)
);
This is kinda explained in the manual:
Normally, it is not necessary to set parameters. But, if you want to send the generated MIME message using Mail then you have to set eol to "\n".
I don't know why this isn't default, but I think it will be backwards compatibility to some early version.