[1.2.X] Fixed #15533 -- Now use 3857 as spherical mercator projection instead...

[1.2.X] 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.

Backport of r15845 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15846 bcc190cf-cafb-0310-a4f2-bffc1f526a37