fix File.executable? when native is disabled

use the new canExecute method from the FileResource to answer the question.
for File#executable_real? use posix if native is available. also added
tests for executable? as well for executable_real?. fixes #2297

Sponsored by Lookout Inc.
Merge branch 'jruby-1_7' of github.com:jruby/jruby into jruby-1_7