Nil-out @collections so #collections will re-compile

lib/jekyll/site.rb

@@ -55,7 +55,7 @@ module Jekyll
       self.pages = []
       self.static_files = []
       self.data = {}
-      @collections = {}
+      @collections = nil
 
       if limit_posts < 0
         raise ArgumentError, "limit_posts must be a non-negative number"