Fixes #1156 Liquid::Utils has no tests

[alebruck]

Fixes #1156

Added tests to Liquid::Utils, tried to understand the context and constraints of each one of the methods based on the use cases.

Didn't add a test for #slice_collection as it seems to be a workaround created for ForTagTest.

I've also added a skip for test_to_date_with_string as it seems like minitest is failing to stub the timezone offset, I'm investigating it further on minitest.

[alebruck]

CLA done

[alebruck]

@shopmike What's your opinion on adding https://github.com/travisjeffery/timecop to the test dependencies? I'm struggling to test time methods that depend on current time/zone and timecopwould make it easier.

[shopmike]

Yeah, I think it is worth trying and seeing what it looks like. Just make sure it's only loaded for tests, so it can't impact Liquid users.