Writing public://composer/composer.json permissions not correctly checked

On my dev boxes I use to own files with my user and group www-data, so I can run drush without sudoing.

When installing composer_manager I got 'RuntimeException' with message 'public://composer/composer.json is not writable, but the file was writable. Tried removing it so I could regenerate it, same problems.

is_writable() php function does not verify all the possible conditions. In core, file_save_htaccess and similar functions rely on the return of file_put_contents, maybe we should do the same here.