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 do believe that in most cases multiple ways of getting the object type will work.
As for this...`response_to?` makes it (imho) more duck-type-like than just checking the type of object.
You dont really get advantages from `respond_to?` in this case.
Nevertheless `to_str` and `to_ary` are no methods, they are symbols for the `respond_to?` method.