Fix for document filter when the keywords param is an array
Fix for document filter when the keywords param is an array

shouldn't happen but maybe scrapers are doing this.

ggoral/whitehall,askl56/whitehall,ggoral/whitehall,alphagov/whitehall,askl56/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall,YOTOV-LIMITED/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,ggoral/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,YOTOV-LIMITED/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,alphagov/whitehall,hotvulcan/whitehall,askl56/whitehall,alphagov/whitehall,askl56/whitehall