[wsgi] make sure request.content is available for consumption
The wsgi.input stream is not seekable, so make a copy either to memory
or disk so it's still available to the user even after we've parsed it
(for POST form processing). Closes #3554996
../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc