Make IdpMetadataParser#get_idp_metadata public

[calebhearth]

I have a use-case for being able to cache the intermediate fetch of
metadata in case of temporary failures, so rather than:

parser.parse_remote(url)

I'd like to

begin
  metadata = parser.get_idp_metadata(url, true)
  do_my_caching(metadata)
  parser.parse(metadata)
rescue HttpError
  load_cache
end

There's a fair amount of logic in the get_idp_metadata method that I'd
rather not need to re-implement. Right now I have this implemented with
parser.send(:get_idp_metadata, url, true) which is obviously not great
if the internals of this class change in the future. Can we move this
method to the public API?

[johnnyshields]

Seems reasonable. @pitbulk I vote to merge.