Commercetools

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- Implement testing/introspect token endpoint, thanks to mbarga!

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- Small fixes if data was not initialized for mock shipping methods actions

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- Implement AddZone and RemoveZone actions for shipping methods

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- Added discount codes
- Added cart discounts

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- Fix small bug in reference expansion if value is None

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- Generate with new API definitions, biggest changes are Resource -> BaseResource and ResourceSchema -> BaseResourceSchema
- Add resource expansion to all applicable objects, see https://docs.commercetools.com/http-api#reference-expansion for details.