Content-type does not handle the case of a linefeed: "The Content-Type of your message is not supported by PHPMailer"

Content-type can optionally have a return in it and often does:

Content-Type: multipart/alternative;
	boundary="----=_NextPart_000_000B_01CD9A85.2A6D33A0"

However, this results in SMTP rejecting the content type as unknown because it breaks after the ';' only with a following space.

The result of this behavior is described in the htmlmail issue #1339276: Cannot get HTML mail with SMTP Authentication and Mail MIME modules enabled - a message that says "The Content-Type of your message is not supported by PHPMailer and will be sent as text/plain instead"

It's a simple fix; patch attached.