[nmh-workers] Can't forward MIME-encoded message

After upgrading to nmh v1.6, I can no longer forward a message with a 
content-type header.

For instance, if the message header contains

Content-Type: multipart/alternative;
        boundary="----=_Part_1122334455"

and the body of the message contains

------=_Part_1122334455
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

------=_Part_1122334455
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

nmh will completely remove the "Content-Type" in the header, so that the 
message is just garbage and cannot be read.  (It leaves the two "Parts" in the 
body though.)

I suspect this is the result of "mhbuild" now being unconditionally inflicted 
on the message, but I can't find any way to disable it, in order to test that.

My nmh release came with Debian version 9.


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