fix: buying a pickup when providing a rate

[Justintime50]

Description

• Fixes how params are passed to the API when buying a pickup and providing a pickup rate (closes [Bug]: pickup.buy(String, PickupRate) wrong params in POST request #340)
  • Removes the unusable buy a pickup overload where no params are specified as carrier and service are required paramaters when buying a pickup

Testing

• Updates tests to pass proper params

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)