respect auth_token=NULL when used in install_github()

[eeholmes]

#818 SAML sign-on behavior with GitHub Enterprise makes install_github() impossible to use for GitHub repos on Enterprise accounts with multiple orgs. Users on one org will be unable to install R packages in public repos on other orgs in the Enterprise account. If remotes would respect auth_token=NULL then that can be put in the install instructions.

Currently when you use install_github(..., auth_token=NULL), remotes does not fully respect the 'request' not to use a PAT and not to use github_pat(). The call to github_pat() (instead of using NULL that was passed in) happens in 2 places.

The changes suggested cause auth_token=NULL to be used consistently, as if there were no PAT at all (which is what is being requested.

Since NULL is what github_pat() returns if there is no PAT stored, it seems like this does not break anything. It simply respects auth_token=NULL.

[gaborcsardi]

Thanks! I am not entirely sure if that functions you changes are called from another code path or not. E.g. during update(pkg_deps()). If they are, this PR potentially breaks those usages if they rely on the auth token being filled in when NULL.