Update simulateTransaction Requests to Send Parameters as Object

[stellarsaur]

Whenever we're sending simulateTransaction JSON RPC requests, we're sending the parameters as an array. However, if we add new fields to a SimulateTransactionRequest that should be optional (this PR for context), we will need to always pass those new fields in the request. However, we can omit parameters if we use postObject to call the simulateTransaction endpoint.