zope5: Since 5.8.1 missing Content-Type HTTP header is interpreted as application/x-www-form-urlencoded (!1751).

Zope.git:
  commit 5b324f6c461f5ea1cc069739b6c32a1a5ff59df9
  Date:   Thu Jan 19 07:15:18 2023 +0100
    replace `cgi.FieldStorage` by `multipart` (#1094)
    * interpret a missing `CONTENT_TYPE` as `application/x-www-form-urlencoded`

With cgi.FieldStorage, it was interpretated as text/plain so Content-Type has to
be provided now (even for WebDAV despite RFC 4918 stating that it *SHOULD* be
provided).

Some Unit Tests did not provide such header at all but this was wrong (such as
erp5_stripe:testStripe where the real request has application/json as
Content-Type and not text/plain).