Fixed #15533 -- Now use 3857 as spherical mercator projection instead of...

Fixed #15533 -- Now use 3857 as spherical mercator projection instead of deprecated 900913 on platforms that support it.  Thanks to cnorthwood for bug report and Christopher Schmidt for OpenLayers JavaScript workaround.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15845 bcc190cf-cafb-0310-a4f2-bffc1f526a37