WEBrick serving SVG under https

I have been developing some interactive SVG running under WEBrick.
Works fine until the site goes under SSL. At that point my Javascript
code fails to return a document using either contentDocument, or
getSVGDocument(). I don't really understand the fix, but following a
hint from a list back in 2007, everything works again when i put the
following line in httputils.rb. On my installation that is at


      "svg" => "image/svg+xml",

added into the object DefaultMimeTypes.

Here is the header to httputils.rb