Ruby 2.6 introduces `Enumerable#filter` as an alias to
`Enumerable#select`, and so our Filterable.filter method will not work
with Ruby 2.6.
So we're renaming the method to `filter_by`, which is similar to
`find_by`. We could also change the `filter` method so if a block is
given it delegates to `Enumerable#filter`, the same way ActiveRecord
handles the `select` method, but IMHO this is easier to follow.
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