Content-Transfer-Encoding header within multipart messages not parsed properly

When sending a "multipart/alternative" message where the HTML portion is base64 encoded, the Content-Transfer-Encoding is not set properly if there is no space in the "Content-Transfer-Encoding" header after the colon.

For example, sending a message with the following body will NOT mark the section with base64 encoding as being base64 encoded, so the raw base64 text will be displayed in the recipient's email client:

This is a multi-part message in MIME format.

--9a8f7e25b7f276955e4f41094c87281db5db920e1
Content-Type:multipart/alternative;
 boundary="0e8885444c69ad0beca8e4c92eb849283046ce530"
Content-Transfer-Encoding:8bit


--0e8885444c69ad0beca8e4c92eb849283046ce530
Content-Type:text/plain; charset=utf-8
Content-Disposition:inline
Content-Transfer-Encoding:8bit

[SOME TEXT]

--0e8885444c69ad0beca8e4c92eb849283046ce530
Content-Type:text/html; charset=utf-8
Content-Disposition:inline
Content-Transfer-Encoding:base64

[SOME BASE46 ENCODED CONTENT]

--0e8885444c69ad0beca8e4c92eb849283046ce530--

--9a8f7e25b7f276955e4f41094c87281db5db920e1--