Fix: Make sure to dup Array in Channel.select_impl

[straight-shoota]

This patch fixes a bug in Channel.select_impl. The method duplicates the incoming collection ops as ops_locks for sorting. ops is an Indexable and can be an Array. Using #to_a is insufficient for ensuring duplication because Array#to_a returns self.
As a result, the original ops would be modified through sorting if its an Array. This is an unintended side-effect.
Furthermore, the returned index which indicates the position of the action that was matched would correspond to the index in the sorted array and thus not correspond to the original indices.

This bug probably does not have a huge effect unless you're using the internal Channel.select API directly. Public uses are for the implementation of the select keyword, but that's using a Tuple type.
Array is typically used as a parameter to Channel.receive_first and Channel.send_first, though. Those methods do not return the index (ref #10411). The side-effect of mutating is also irrelevant because the list of actions is only created in the implementation of these methods from a list of Channel.

For the latter use case, there is no need to duplicate the array. So I think we could introduce an optimization to avoid that, by extracting from select_impl a method which receives both ops and ops_locks (which can the point to the same collection).