Fixed handling objects as API input parameters

[daniellockyer]

fix https://linear.app/tryghost/issue/SLO-155/paramsmap-is-not-a-function-an-unexpected-error-occurred-please-try

• in the case you provide an object to the API, this code will throw an error because it can't map over an object
• we can just assert that params should be an array and throw an error otherwise