Generate download link for specs defined by an object

[seanpk]

If I make use of the "advanced feature" to provide an object to Redoc.init I lose the "Download OpenAPI specification: Download" download button.

e.g.:

<redoc></redoc>
<script>
  var spec = { /* my spec as an object */ };
  Redoc.init(spec);
</script>

ReDoc should still provide the Download link in this case and generate a JSON file by stringifying the object provided.