Validate that HTTPS is used by default for API requests

Since we use a test server representing the GitHub API, all requests get
transformed to HTTP in tests. Make a check that the original requests
really was intended to go over HTTPS.
Support Enterprise over HTTP via `protocol` config option

Example `~/.config/hub`:

  my.example.org:
  - user: myuser
    oauth_token: mytoken
    protocol: http

Currently there is no way to set `protocol` other than manually editing the
configuration file.
Make `browse` and `compare` respect `protocol` config option

Allows using these commands for Enterprise configurations over regular HTTP.
Merge pull request #488 from github/enterprise-http

Allow Enterprise configured for regular HTTP