Call a cloud function with a master key

[junpayment]

In the following code, useMasterKey () always returns false, so cloud function cannot be used with master key.

https://github.com/kylemcc/parse/blob/428efbfdaa1d8231e9b7c159f7d3c630e7c5b682/session.go#L187

func (s *loginRequestT) useMasterKey() bool {
	return false
}