Prevent duplicated querystring parameters when getting batches of contacts

[sangaline]

The ContactsClient.get_batch() method was mutating the ContactsClient.default_batch_properties when adding in the extra_properties. This resulted in a situation where subsequent calls to ContactsClient.get_batch() would cause property=X to be repeated once for each time it had been included previously. This eventually leads to a bad request error from Hubspot once the URI gets too long.

This PR stops the ContactsClient.default_batch_properties mutation from happening, and uses a set instead of a list so that there won't be duplicate properties in the query string even if they appear in both extra_properties and ContactsClient.default_batch_properties.

[jpetrucciani]

Another good catch! I'll get this merged and released soon.

[jpetrucciani]

This is now live on pip as version 3.2.23!

Thanks again for this fix! 😄