Added Sum ArrayFilter with 2 different options

[cpriestland]

Added "Sum" ArrayFilter so that others can use it also.

2 options:

• Can include an argument to indicate which member to Sum on an array of objects
• No argument which will attempt to Sum the array items value

Tests included with Pull Request which check both of these scenario's

We use Fluid quite a lot at TemplateTo and hope you don't mind us putting our link in your UsedBy section, Fluid can be used by our users as well when building out their templates.

[cpriestland]

@hishamco Thanks for approving this, is there an idea of when this could be merged as we require the functionality? Thanks

[sebastienros]

Would you mind adding the same tests as in Shopify Liquid to ensure everything behaves as the specs?
https://github.com/Shopify/liquid/blob/f5ed5404b5ba04c7010d4f37ca2928bb6fa925a2/test/integration/standard_filter_test.rb#L952-L1052

[cpriestland]

Thanks for your reply @sebastienros I have added the tests as suggested

[hishamco]

Thanks for your contribution @cpriestland