Fixed #18314 -- Corrected request.build_absolute_uri() handling of paths starting with //

``HttpRequest.build_absolute_uri()`` now correctly handles paths starting with ``//``.
``WSGIRequest`` now doesn't remove all the leading slashes either,
because ``http://test/server`` and http://test//server`` aren't the same thing
(RFC2396).

Thanks to SmileyChris for the initial patch.

tests/requests/tests.py

@@ -10,7 +10,7 @@ from django.core.exceptions import SuspiciousOperation
 from django.core.handlers.wsgi import WSGIRequest, LimitedStream
 from django.http import (HttpRequest, HttpResponse, parse_cookie,
     build_request_repr, UnreadablePostError, RawPostDataException)
-from django.test import SimpleTestCase, override_settings
+from django.test import SimpleTestCase, RequestFactory, override_settings
 from django.test.client import FakePayload
 from django.test.utils import str_prefix
 from django.utils import six
@@ -693,3 +693,64 @@ class HostValidationTests(SimpleTestCase):
             msg_suggestion2 % "invalid_hostname.com",
             request.get_host
         )
+
+
+class BuildAbsoluteURITestCase(SimpleTestCase):
+    """
+    Regression tests for ticket #18314.
+    """
+
+    def setUp(self):
+        self.factory = RequestFactory()
+
+    def test_build_absolute_uri_no_location(self):
+        """
+        Ensures that ``request.build_absolute_uri()`` returns the proper value
+        when the ``location`` argument is not provided, and ``request.path``
+        begins with //.
+        """
+        # //// is needed to create a request with a path beginning with //
+        request = self.factory.get('////absolute-uri')
+        self.assertEqual(
+            request.build_absolute_uri(),
+            'http://testserver//absolute-uri'
+        )
+
+    def test_build_absolute_uri_absolute_location(self):
+        """
+        Ensures that ``request.build_absolute_uri()`` returns the proper value
+        when an absolute URL ``location`` argument is provided, and
+        ``request.path`` begins with //.
+        """
+        # //// is needed to create a request with a path beginning with //
+        request = self.factory.get('////absolute-uri')
+        self.assertEqual(
+            request.build_absolute_uri(location='http://example.com/?foo=bar'),
+            'http://example.com/?foo=bar'
+        )
+
+    def test_build_absolute_uri_schema_relative_location(self):
+        """
+        Ensures that ``request.build_absolute_uri()`` returns the proper value
+        when a schema-relative URL ``location`` argument is provided, and
+        ``request.path`` begins with //.
+        """
+        # //// is needed to create a request with a path beginning with //
+        request = self.factory.get('////absolute-uri')
+        self.assertEqual(
+            request.build_absolute_uri(location='//example.com/?foo=bar'),
+            'http://example.com/?foo=bar'
+        )
+
+    def test_build_absolute_uri_relative_location(self):
+        """
+        Ensures that ``request.build_absolute_uri()`` returns the proper value
+        when a relative URL ``location`` argument is provided, and
+        ``request.path`` begins with //.
+        """
+        # //// is needed to create a request with a path beginning with //
+        request = self.factory.get('////absolute-uri')
+        self.assertEqual(
+            request.build_absolute_uri(location='/foo/bar/'),
+            'http://testserver/foo/bar/'
+        )