Don't include CONTENT_TYPE/CONTENT_LENGTH in environ if absent in headers

[thusoy]

This has been reported earlier in #1049 and #1159, but closed since it's valid behavior according to the WSGI spec. It is however inconsistent with how most of the wsgi servers handle it.

With the following test application I tested this for gunicorn, gevent, twisted and uwsgi:

# test_app.py
from werkzeug.wrappers import Response

def application(environ, start_response):
    content_type = environ.get('CONTENT_TYPE')
    response = Response(repr(content_type) + '\n')
    return response(environ, start_response)

Test case	Outcome
gunicorn test_app	None
gevent.WSGIServer.serve_forever()	None
twistd -n web test_app.application	''
uwsgi --http :8000 --wsgi-file test_app.py	None

Of the four tested, only Twisted handles the environ the way werkzeug does, the others will experience inconsistencies between their test and production environments.

We had trouble reproducing a production error in test because of this as it's currently non-trivial to test a missing content-type header in flask. If absent by default it's easy to test both absent and empty header values, as you can send an empty string in the headers to get the old behavior.

[lepture]

It makes sense to me since those common production ready servers behave like this.

[davidism]

I updated this to only omit the headers if they're None, which seems more sensible in terms of being able to test different values.