Bug: fields never set if value is falsy

[longseespace]

Overview

These boolean fields will never be included in the payload if the value is false.

Suggestion: replace if (billImmediately) with if (billImmediately !== undefined)

Reference: https://github.com/avaly/paddle-sdk/blob/55112c748cc30202e43c701146e34178cb80a6e1/sdk.js#L460