keys throws an error if there are no keys in the key store. Fix it so it returns properly if there are no keys.
NoMethodError: undefined method `[]' for nil:NilClass
from /Users/drewb/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/rundeck-0.0.3.pre/lib/rundeck/request.rb:89:in `error_message'
from /Users/drewb/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/rundeck-0.0.3.pre/lib/rundeck/request.rb:39:in `validate'
from /Users/drewb/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/rundeck-0.0.3.pre/lib/rundeck/request.rb:14:in `get'
from /Users/drewb/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/rundeck-0.0.3.pre/lib/rundeck/client/keys.rb:16:in `keys'
from /Users/drewb/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/rundeck-0.0.3.pre/lib/rundeck.rb:22:in `method_missing'
from (irb):29
from /Users/drewb/.rbenv/versions/2.0.0-p247/bin/irb:12:in `<main>'
keys
throws an error if there are no keys in the key store. Fix it so it returns properly if there are no keys.