Fixed #18314.request.build_absolute_uri() finally functions correctly

https://code.djangoproject.com/ticket/18314 This intro ticket is 13 months old. It finally has a PR now.

In "django/http", request.build_absolute_uri() finally functions correctly when path starts with "//". Thanks "SmileyChris" and "yoyoma" for the 2 similar code patches. I manually applied the "18314.diff" patch given by username "SmileyChris". The tests  in "tests/http/" failed before the patch was implemented, and then the tests succeeded after the patch was implemented. Please code review and let me know how I can make this PR better.