Improve where filter tests

[andershagbard]

This makes sure a feature (Which may be caused by a bug) is tested. I've seen this used in a few themes.

This is mainly used on tags within Shopify.

assign tags = product.tags | where: 'some_prefix:'

This will get all tags containing some_prefix:.

[macournoyer]

Oh wow! This works because "beta"["be"] returns a truthy value in Ruby.

But make sense to add a test since it's used in some themes 👍

[dylanahsmith]

Thank you very much for this test. I could definitely see how not having this test could result in a regression from someone changing liquid without considering String#[] being leveraged in this way for the where filter.