Please support allow_false feature

[ns-jjiang]

Checklist

• [x] I have searched the issue list
• [ ] I have tested my example against Shopify Liquid. (This isn't necessary if the actual behavior is a panic, or an error for which IsTemplateError returns false.)

Expected Behavior

Parse {{ display_price | default: true, allow_false: true }} without any error.

Actual Behavior

It shows Liquid error (line 12): syntax error in "display_price | default: true, allow_false: true" in {{ display_price | default: true, allow_false: true }}

The syntax is from the shopify document: https://shopify.github.io/liquid/filters/default/

Detailed Description

Possible Solution

[osteele]

Blocked on #42. There is a PR under development for that issue; once it is merged I will fix this one.