Use array instead of Hash to keep the registered filters

1.8.7 compatibility fix

In Ruby 1.8.7, Hash does not preserve insertion ordering as Array does.
This could cause a problem when registering filters which depend on others and
the registration order is important.

So, the @@filters variable was changed to array where the order of the filters is
the same as the insertion order.

test/liquid/hash_ordering_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+module MoneyFilter
+  def money(input)
+    sprintf(' %d$ ', input)
+  end
+end
+
+module CanadianMoneyFilter
+  def money(input)
+    sprintf(' %d$ CAD ', input)
+  end
+end
+
+class HashOrderingTest < Test::Unit::TestCase
+  include Liquid
+
+ def test_global_register_order
+    Template.register_filter(MoneyFilter)
+    Template.register_filter(CanadianMoneyFilter)
+
+    assert_equal " 1000$ CAD ", Template.parse("{{1000 | money}}").render(nil, nil)
+ end
+
+end