Put the default anchor schemes back in the default whitelist

Clients of this gem may have been relying on this.
Add a test for SanitizationFilter::ANCHOR_SCHEMES

Clients may come to rely on this, so we should have a test to ensure we
don't break them.
Allow specifying anchor schemes in the whitelist itself

If context[:anchor_schemes] is not passed, we use the schemes that are
specified in the whitelist itself. This matches our behavior from before
:anchor_schemes was added.

If :anchor_schemes is passed, it overrides any schemes specified in the
whitelist.