fix: webhook fixture, addCreditCard params

[Justintime50]

Description

1. Uses the new webhook fixture data (had to update escaping logic due to URLs being present, shouldn't be an issue for users since it'll come off the wire instead of from a file
2. Found when re-recording cassettes (they were all expired) that our addCreditCard function was broken and not properly passing URL params. After some research, I realized the Guzzle has this supported natively and that the old urlEncode function we home-built some time ago isn't needed and could be removed

Testing

• Updated tests to match new assertions

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)