Fix Object#bullet_key performance regression

When Array#join is calling in jruby, self.class#to_str is called, which is missing in Class (BasicObject#method_missing is called). 
Thus this call affects performance and tests become up to 3 times slower.
Array#join add additional delay to perform #bullet_key also.
Merge pull request #191 from ingeniarius/master

Fix Object#bullet_key performance regression