I don't know if there might be a better place to report this, but I think there is a bug with the array version of Email::header. The two-parameter version works fine, but the array version gets ignored.
Browsing the code, I suspect packages/email/classes/email/driver.php: in header(), the foreach uses $_header => $_value, but the following line uses $header / $value (line 455 in v1.5.3).