.httr-oauth file should be created with perms 0600 by default

[craigcitro]

Title says it all -- I can't see a reason that we shouldn't default to creating the file with restrictive permissions. I'm happy to send a PR if folks agree.

[hadley]

PR would be awesome :)

[jennybc]

Could this make it hard to read .httr-oauth in some Shiny situations?

[daattali]

I agree, I also think this is going to introduce bugs to shiny users.

For example, shiny apps on a shiny server by default run as the "shiny" user, who is normally not the owner of the file, so shiny apps will get permission denied issues. Is this correct, or am I missing something?