Custom (un)marshalling for CustomsItem to account for Value type

[nwithan8]

Description

• Custom (un)marshal functions for CustomsItem to account for float/string discrepancy with Value
  • Value will always be a string when serialized to JSON
  • Value can be either a string or a float when deserializing from JSON
• Remove workaround from fixtures
• Re-record cassettes as needed

Testing

• Manually tested and confirmed that deserializing a string (via API response) and a float (via fixture JSON) both work
• Cassettes re-recorded as needed
• All unit tests pass

Pull Request Type

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

• [x] 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)
• [x] Improvement (fixing a typo, updating readme, renaming a variable name, etc.)