fix: Better check of RPC type requests (#6927)

guessIsRPCReq() considers all POST requests as RPC but doesn't
check if this is an object operation API or not, which is actually
confusing bucket forwarder handler when it receives a new multipart
upload API which is a POST http request.

Due to this bug, users having a federated setup are not able to
upload a multipart object using an endpoint which doesn't actually
contain the specified bucket that will store the object.

Hence this commit will fix the described issue.