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'd rather have this check if it's a String, instead of if it responds to :to_str; the contract (if there even is such a thing) on what `to_s` means vs `to_str` is not really clear in the docs and certainly not clear to me as a reader :P 

I'll change this post-merge.