install_github doesn't properly install from pull requests

[wch]

There seem to be two different bugs here. Examples:

Using the pull= argument:

> devtools::install_github('rstudio/shiny', pull=342, ref=NULL)
Error: client error: (403) Forbidden

Using the user/repo#pull syntax:

> install_github('rstudio/shiny#342')
Error in FUN("rstudio/shiny#342"[[1L]], ...) : 
  Must specify either a ref or a pull request, not both. Perhaps you want to use 'ref=NULL'?

[krlmlr]

The first issue seems to be a temporary one, I can install using your command. The second issue has been fixed (but not yet merged) in #390.

Also related: #388 (where the second issue has been reported first).

[krlmlr]

I can now replicate the first issue in a "clean" environment (from packrat), not sure why it does not occur in my "regular" environment. Can be fixed by passing

, config = add_headers("User-agent" = "hadley/devtools")

to the GET call in github_pull_info. I'm about to submit a pull request.