Forward params on pagination

[ralexmatthews]

Description

There was an issue where sometimes when fetching the next page, we wouldn't have the original params sent along. So this stores the data used when fetching the next page in a new key on the EasyPostObject, _params, that is then added when fetching the next page.

Testing

I ran the test suite and it worked, with some modifications to make sure we can ignore the _params when comparing input to output.

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[nwithan8]

Paired with Alex on this, can confirm logic seems sound, approach actually makes it a bit more future-proof and resilient while also cleaning up bad automagical stuff.