Add support for redirects

[vadeg]

When the DefaultURIDownloader tries to load a schema and got a response 301 Moved Permanently it tries to parse the body content and fails (For example check zalando/zally#1164).

Would be great if the DefaultURIDownloader could handle redirects and instead of failing, try to download schema again.

[sdoeringNew]

The DefaultURIDownloader generally follows redirects. It does not follow redirects if the schema changes. Your example redirects from http://swagger.io/v2/schema.json# to https://schema.swagger.io/v2/schema.json. Therefore it fails.

And that behaviour is okay, see: https://stackoverflow.com/questions/1884230/httpurlconnection-doesnt-follow-redirect-from-http-to-https