Refactor REST_Controller::_parse_options()

[kenjis]

See #544

[chriskacerguis]

This doesn't seem to account for CLI input. Thoughts?

[kenjis]

See my Rest_test controller: https://github.com/kenjis/ci-app-for-ci-phpunit-test/blob/fda2365f7887daefc1122a9bd11980c9d86b05e7/application/controllers/api/Rest_test.php

Before this PR:

$ php public/index.php api rest_test index abc def
{"get":{"abc":"def"},"query":[]}

This PR:

 php public/index.php api rest_test index abc def
{"get":{"abc":"def"},"query":[]}

The resutls are the same.

Is there any way to specify HTTP method via CLI?