Comply with current umask when generating new app:

  When creating a new application using the default rails generator
(`rails new my_app`), it will apply some permissions bypassing the
current process umask. The `bin' directory and any files inside it will
always be set a mode of 0755.

  This change removes the current umask bits from the mode applied by
the generator on the `bin' directory and its content.
Merge pull request #12048 from tjouan/app_generator-bin_perms-umask

Comply with current umask when generating new app