Route#duplicate: Use `#slice!` instead of `#slice`

Johan suggested simplifying this method by changing the `slice` call:
http://api.rubyonrails.org/classes/Hash.html#method-i-slice-21

I had modified this to fix the whitespace while working on the `WayCost`
calculation function for `Chouette::Route`s, so don't really have a
brain cache hit on this code, but the change makes sense and seems to
work.

Refs #6095

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