Allow relative URI in redirection

RFC 7231 allows for relative URI in the redirection response: https://tools.ietf.org/html/rfc7231#section-7.1.2

However, WebOb converts relative URI to absolute before sending the response. It is done when using HTTPFound, HTTPMovedPermanently exceptions or when using Response with Location header.

Here are pieces of code which demonstrate that. This code is not using any web server. It is pure Python WSGI reference implementation

from wsgiref.util import setup_testing_defaults
from wsgiref.simple_server import make_server

from webob import Response

def app(environ, start_response):
    headers = [('Location', './new_page')]
    r = Response("redirected", status=301, headers=headers)
    return r(environ, start_response)

httpd = make_server('', 8000, app)
httpd.serve_forever()

from wsgiref.util import setup_testing_defaults
from wsgiref.simple_server import make_server

from webob.exc import HTTPMovedPermanently

def app(environ, start_response):
    r = HTTPMovedPermanently(location='./new_page')
    return r(environ, start_response)

httpd = make_server('', 8000, app)
httpd.serve_forever()

Start with the Response handling for a Location header and the HTTPMovedPermanently exception, using the two WSGI examples as reproduction cases. Trace where a relative './new_page' is converted before the response is sent. Done means relative redirect URIs remain relative in both cases, while absolute URIs continue to work.