allow "http://" type Github hosts, not only "https://"

[ampu3ro]

My company has an enterprise account with root endpoint http://github.bdap.enernoc.com/api/v3 for the API. I would imagine some others have hosts like this as well. In fact, Github's manual refers to endpoints as http(s)://.

The minor tweak in the PR defaults to https: if not specified in the host argument, otherwise allowing something like

install_github("me/myrepo",host="http://github.bdap.enernoc.com/api/v3")

to work. Hope it helps!

[ampu3ro]

PTAL. Sorry about the unnecessary commenting :/ first time

[jimhester]

This PR breaks the tests on linux and windows. I would suggest running the tests locally with devtools::test() and figuring out why this change breaks them, then update your code so it no longer does so.

[ampu3ro]

Can't say it was easy (rtools is super finicky!), but I think I got all the tests to pass. Not sure what's failing in the AppVeyor tests though...

[ampu3ro]

much better solution. thanks