Fix tests for Ruby 1.9.3 from 3e13bf3

Changes in 3e13bf3 broke tests for Ruby 1.9.3 which doesn't support
Enumerable on Strings. Workaround this by casting everything as an array and
flattening to prevent existing arrays from being encapsulated.

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/cc @kbarber

Would you prefer to test `.is_a?(Array)`?