Ensure sum filter calls to_liquid on evaluated property value

[adamklingbaum]

Consider an array, products, in which product.price for each product returns a Liquid object that represents the product's price.

Current behaviour:

{{ products | map: 'price' | sum }} returns the correct sum of the prices.

{{ products | sum: 'price' }} returns 0.

Expected behaviour:

{{ products | map: 'price' | sum }} and {{ products | sum: 'price' }} return the same value, the correct sum of the prices.

Explanation of current behaviour

TLDR: map calls to_liquid on the values resulting from property lookup, while the current implementation of sum does not.

The current implementation of sum calls Utils#to_number on the value resulting from each property lookup. If the value is a Liquid object, Utils#to_number will return 0 (unless the object responds to :to_number).

The map filter sends :map (which uses :each) to an instance of InputIterator, whose implementation of each sends :to_liquid to each element before yielding.

So, for Liquid objects who respond to :to_liquid with a numeric, the elements of {{ products | map: 'price' }} will be the result of :to_liquid for each element, and the sum will return the expected value. This same :to_liquid parsing does not currently happen in the sum filter.

Proposed change

To make this filter consistent with others, such as map, if the value of item[property] responds to to_liquid, it should be parsed to liquid before being coerced into a number and summed.

Proposed logic for the with-property branch of StandardFilters#sum:

1. Construct an array, values_for_sum, which is the result of mapping each item to item[property].
2. Decorate values_for_sum as an InputIterator, since InputIterator#each sends to_liquid to any element before yielding the element.
3. Return the sum of Utils.to_number(item) for each item of InputIterator.new(values_for_sum, ...).

With this change, I have also refactored to make the method more concise and included additional tests for the expected to_liquid behaviour.