Steps to reproduce:
setfacl -R -m u:`whoami`:rwX /opt/rubies/rbx-2.2.5/gems
File.writable?('/opt/rubies/rbx-2.2.5/gems') # => false, while MRI returns true.
I use setfactl to grant users in rubygems group write ability to rubygems directories. This issue makes bundler believe I do not have writable access and ask sudo password every time.
If people are busy with other issues, I can try to look into this when I am free (probably several weeks later).