skip original Content-Transfer-Encoding for mbox

skip original Content-Transfer-Encoding for mbox

In the commit 01b9cbb9 all original mail headers are copied into the
resulted mbox file. This means that some headers are going to be
present twice in the generated mbox. That is fine unless the original
email arrived in base64 encoding.

Apparently git relies on the latest Content-Transfer-Encoding key. And
since downloaded patch's actual encoding is '7bit', git fails to apply
it with the message 'Patch is empty'.

Since patchwork adds a proper 'Content-Transfer-Encoding' anyways,
let's skip this field while copying headers from the original mail

Signed-off-by: Yuri Volchkov <yuri.volchkov@gmail.com>
Acked-by: Veronika Kabatova <vkabatov@redhat.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>