Fix tests for #438

Test for permissions before overwriting with new data to prevent
disclosure of a new key.

Test permissions using integer instead of string comparison.

Use alert_error and terminate_interaction (RubyGems API) instead of
abort.
Removed duplicate rubygems_api_keys= tests
Enforce credential permissions on read and write

Writing the credentials was covered but not reading them.

Updated wording to make links easier to copy and paste.

Suggested the user "should" reset their credentials if there is a
possibility of disclosure.

This completes #438.