Added type checks for cpan authors

CPAN 'authors' can be a list of strings, just a string or not set at all. This adds type checks to handle the different possibilities. Related to #712
I don't think there's any strict rule to play here, Ruby's a very flexible language in this respect. My personal habit is to either check for type or check for a method. Oddly, I've never seen this 'to_ary' and 'to_str' methods used in 7 years of ruby programming. Weird! :P