Pass through options on assign tag

@fw42 

This fixes a bug @fw42 found in Shopify/shopify#25531 where even when you set Liquid to strict parsing mode it doesn't catch syntax errors in `assign` tags. This was caused by the assign tag not passing down the options to it's variable.

I searched the codebase for other instances of this problem and didn't find any.

```
[4] pry(main)> Liquid::Template.parse("{% assign test = ('x' | md5) %}", :error_mode => :strict).render
Liquid::SyntaxError: Expected dotdot but found pipe in "{{('x' | md5) }}"
from /Users/tristan/WorkBox/Dev/liquid/lib/liquid/parser.rb:16:in `consume'
[5] pry(main)> Liquid::Template.parse("{% assign test = ('x' | md5) %}", :error_mode => :lax).render
=> ""
```