getRelationship method gets called twice per each resource in response

[bebor]

suppose we have a request url http://xxx/jsonapi/speakers/1654?include=sessions

the result will be one primary resource of speakers, and it will include sessions, which will come from (in this test scenario) from SpeakerSerializer->sessions()

but that sessions() method gets called twice (and it should be called only once, since there is only 1 primary element). if the request would call for a list of speakers, like so: http://xxx/jsonapi/speakers/?include=sessions, then the SpeakerSerializer->sessions() will get called twice the time of the count of the speakers items

i debugged the code, and found that the SpeakerSerializer->sessions() gets called:

1. here
   AbstractSerializer.php
   line 75, inside of public function getRelationship($model, $name)

$relationship = $this->$method($model);

this AbstractSerializer.php:getRelationship() gets called from Resource.php:toArray()

2. and again the same line (meaning AbstractSerializer.php:getRelationship()) will be called from Document.php::toArray() from within the line saying:

$resources = $this->getIncluded($this->data);

so to sum it up:
Documen.php:toArray() calls the same code twice per each resource, and does so on the line saying
$document['data'] = $this->data->toArray();
and just next to it
$resources = $this->getIncluded($this->data);

[tobyzerner]

Thanks for the detailed report @bebor. I've pushed a fix, can you confirm it's working as expected?