Site Maps: Generating under HTTPS on a split site leads to wrong URL

This needs testing to confirm but steps to duplicate:

1. Have a site setup with http front-end, https backend.
2. Activate the sitemaps modules to generate a sitemap
3. View the sitemap

Expected:
Everything looks normal.

Actual:
Blank page with a console error of a http/https mixed content error which fails to load the XSL stylesheet.

cc: @ryanmarkel

Start by reproducing the split-site setup with an HTTP front end and HTTPS backend, then inspect the generated sitemap and its XSL stylesheet URL. Done means the sitemap loads without a mixed-content console error and the stylesheet uses the correct URL scheme.

Written by the indexing model from the issue text.