Revert "Raise FilterNotFound on use of non-existent filter"

This reverts commit 01c25a11a3.

Conflicts:

	test/context_test.rb

lib/liquid/context.rb

@@ -57,7 +57,7 @@ module Liquid
       if strainer.respond_to?(method)
         strainer.__send__(method, *args)
       else
-        raise FilterNotFound, "Filter '#{method}' not found"
+        args.first
       end
     end