Fix stash_drop of S4 class git_stash object

* R/stash.r: Used old name 'stashes' instead of 'stash_list' to
  determine index of git_stash object

* tests/stash.R: Add test case

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add example to stash_drop method

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>