\Apigee\Mint\Developer::getPrepaidBalance() incorrectly parse response as \Apigee\Mint\DeveloperBalance

[mxr576]

/v1/mint/organizations/[ORG]/developers/[DEVELOPER]/prepaid-developer-balance returns partial information compared with /v1/mint/organizations/[ORG]/developers/[DEVELOPER]/developer-balances (ex.: recurring payment settings are not available in the first response) but current implementation casts the response to the same object as it would be the second API's response and fills missing property's value with the default value of the property. Consequently, the first API call always returns false positive information about developer's recurring payment settings (isRecurring, recurringAmount, etc.) in the SDK.