use URI::HTTPS to generate HTTPS redirects

[cgc]

When deploying my app, I noticed the callback URL (after this redirect) was stalling, trying to load http://my-app.herokuapp.com:443/api/auth/twitter/callback. Playing with URI::HTTP.build and URI::HTTPS.build, I noticed the scheme kwarg is ignored

> URI::HTTP.build(scheme: 'http', host: 'hi.com', port: 80).to_s
=> "http://hi.com"
> URI::HTTP.build(scheme: 'https', host: 'hi.com', port: 443).to_s
=> "http://hi.com:443"
> URI::HTTPS.build(scheme: 'http', host: 'hi.com', port: 80).to_s
=> "https://hi.com:80"
> URI::HTTPS.build(scheme: 'https', host: 'hi.com', port: 443).to_s
=> "https://hi.com"

It seems like the best solution is to switch which build() we are using based on the request scheme. I modeled my change after this PR fog/fog#2159 which seems to be the same issue. I was unable to write a test because I don't think request_via_redirect or get_via_redirect supports having a scheme like https.

[booleanbetrayal]

LGTM! Will look at coverage after it lands. Thanks @cgc !