ApiSetup: implement defaultIncludeFields setup option

[nathanuphoff]

An Api can be constructed with a defaultIncludeFields setup option, its value can be either "none" (default) or "primary". The option specifies whether 'primary' relationships should be included by default but right now it does nothing.

For this issue to be resolved the jsonapi-client must:

• ApiController#decodeResource must parse included resources by default for each primary relationship if the setup option is set to "primary",
• the FilteredResource generic must conditionally alter its return type following the setup option.