Made ActionView::MissingTemplate#initialize optionally accept a single string

prefix to be converted to an array as in ActionView::PathSet#find_all.

actionpack/lib/action_view/template/error.rb

@@ -29,6 +29,7 @@ module ActionView
 
     def initialize(paths, path, prefixes, partial, details, *)
       @path = path
+      prefixes = [prefixes] if String === prefixes
       display_paths = paths.compact.map{ |p| p.to_s.inspect }.join(", ")
       template_type = if partial
         "partial"