swift4: handle collection params in path

[jacobweber]

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Description of the PR

Handle path params that are collections in swift4, for #2125.

Assumes the collectionFormat is CSV for now, although that could be added as a parameter.

I'm not very familiar with Swift, so please let me know if this could be improved.

[jacobweber]

Just to clarify: my change only supports CSV for path parameters, because the existing code only supports CSV for query/form parameters. I didn't want to change too much. But ideally it would support other formats for any parameter type.

[wing328]

The change looks good to me. If no further question, I'll merge it tomorrow (Wed)