`utils.http.same_origin` doesn't comply with RFC6454

[Lukas Klein]

According to RFC6454 (​http://tools.ietf.org/html/rfc6454#section-3.2.1) this should both be true:

>>> from django.utils.http import same_origin
>>> same_origin('http://google.com', 'http://google.com')
True
>>> same_origin('http://google.com', 'http://google.com:80')
False

Quote:

All of the following resources have the same origin:

​http://example.com/
​http://example.com:80/
​http://example.com/path/file

Each of the URIs has the same scheme, host, and port components.

Django's same_origin uses the standard urllib, which will return an empty port if none is explicitly specified.

My suggestion (see GitHub pull request) is to extend same_origin to use a protocol-to-port-mapping if no port is explicitly declared.