Add support for clearing relationships

[emil-nasso]

If you sent a request, trying to clear a relationship (remove the relationship) by sending "relationshipname" : { "data": null}" yin ignored this, behaving exactly like it did when the relationship was completely omitted.
The hydrator for the relationship was not called and calls to $jsonApi->getRequest()->getResourceToOneRelationship('relationshipname') behaved just as if no relationship was sent.
This patch intends to change that by handling the null-case and creating empty ToOneRelationships where appropriate.

A request for clarification of this behavior has been made to the folks over at jsonapi, here: json-api/json-api#1070

[kocsismate]

You did a great job with your PR and I couldn't find anything which needs fixing (apart from some really minor formatting things)! :) I'll gladly take care of updating the documentation, change log and tests this evening, but I won't stop you here if you want to do them.

And I think the best thing would be to throw exceptions when either the type or the id field is missing from the resource identifier object, because the specs explicitly declares that they are required: http://jsonapi.org/format/#document-resource-identifier-objects For this purpose a new ResourceIdentifierTypeMissing and a ResourceIdentifierIdMissing exception could be used via the ExceptionFactory. If you also think that this is the best solution I will make this change too.

Furthermore, I noticed one thing I don't like: it is the names of the getResourceToOneRelationship() and getResourceToManyRelationship() methods. They are really awkward and I can't remember why I named them like that. :) But what would you think if I renamed them to getToOneRelationship() and getToManyRelationship()? And maybe the JSON API specific functionality should be separated from the generic RequestInterface to a JSON API specific class. And probably one could access them the following way:

$authors = $jsonApi->getRequest()->getJsonApi()->getToOneRelationship("authors");

But this is really long a line :S But if you have a good solution, feel free to tell me :)

[emil-nasso]

+1 on the exception suggestion and the renaming. :) The exception thing shouldn't be a breaking change, i think, but the renaming of the methods are.