Fixing creating PPE sandbox subscriptions

[dparker1005]

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Changes proposed in this Pull Request:

Sending a content type of application/json has started throwing "malformed JSON" errors when creating a PPE subscription in sandbox mode. This PR changes the content type to application/x-www-form-urlencoded, which should be the correct content type anyways.

Also fixes a PHP warning PHP Deprecated: Implicit conversion from float 1.1274959045124686 to int loses precision

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Changelog entry

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