fixed an attempted join on a string.

Liquid Exception: undefined method `join' for #<String:0x000000026d5f68> in index.markdown
[...]/plugins/citation.rb:31:in `render'
Merge pull request #1 from dmayer/master

fixed an attempted join on a string

Liquid Exception: undefined method `join' for #<String:0x000000026d5f68> in index.markdown
[...]/plugins/citation.rb:31:in `render'