Fix filter parser regex for filter args without separating spaces.

The regex was using \S+ to match the comma between the filters
arguments, but would continue to match idependent quote characters and
filter separators. This can result in multiple filters being interpreted as
a single one with many arguments.

lib/liquid/variable.rb

@@ -11,7 +11,7 @@ module Liquid
   #   {{ user | link }}
   #
   class Variable
-    FilterParser = /(?:#{FilterSeparator}|(?:\s*(?!(?:#{FilterSeparator}))(?:#{QuotedFragment}|\S+)\s*)+)/o
+    FilterParser = /(?:#{FilterSeparator}|(?:\s*(?:#{QuotedFragment}|#{ArgumentSeparator})\s*)+)/o
     attr_accessor :filters, :name
 
     def initialize(markup)