display multiple authors better.

Before:
Copyright (c) 2013 ["a", "b"]

After:
Copyright (c) 2013 a, b
"spec.author" is better for single-author gems.

"spec.author=" expects a String represents a single author.
"spec.authors=" expects an Array which is a list of multiple authors.

http://guides.rubygems.org/specification-reference/
resolve conflict