plugin: fix tests failures on ruby 1.8

Ruby 1.8:

>> :bar_api =~ /_api$/
=> false
>> :bar_api.to_s =~ /_api$/
=> 3

Ruby 1.9:

>> :bar_api =~ /_api$/
=> 3
>> :bar_api.to_s =~ /_api$/
=> 3

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
Merge branch 'master' of github.com:killbill/killbill-plugin-framework-ruby