fix sending fax, when to is multi-value

[Alex-query]

Current version works with only one phone number.
if you try to send fax by a few numbers , server returns 400 Bad Request.
My pull request fixes it , and you can send fax by a few numbers

[grokify]

Thanks for the pull request, but the way it's written, it may be overwritten by future changes.

This code is auto-generated by OpenAPI Generator using the RingCentral OpenAPI spec, so changing one or both of those is the way to update this in a way that it won't be overwritten in the future.