bin/55159: send-pr scrambles PRs

[schaecsn%gmx.net@localhost]

>Number:         55159
>Category:       bin
>Synopsis:       send-pr scrambles PRs
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Apr 10 06:30:00 +0000 2020
>Originator:     Stefan Schaeckeler
>Release:        9.0
>Organization:
>Environment:
NetBSD XXX.XXX.XXX 9.0 NetBSD 9.0 (GENERIC) #0: Fri Feb 14 00:06:28 UTC 2020  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64

>Description:
Just the other day, I was using send-pr for the first time. It scrambled my PR by encoding some characters like = and hard-breaking lines at column 75.

See here http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=55157

I use NetBSD 9 and its postfix.
>How-To-Repeat:

>Fix:
Manually adding this line to the header makes PRs arrive as is:

Content-Type: text/plain