Doing seek(0) on the wsgi.input object in mod_wsgi (3.4) does not work
as there is no such method. Instead, read all the input and then
create a StringIO with that input and pass it to the wrapped app as
its wsgi.input.
Use seek() on wsgi.input if available; otherwise, rewind input stream
by copying it.
Handle the case where CONTENT_LENGTH is not set in the environ.
Changelog for PR #5.