Support creating polymorphic has-many relationships

[saravanak]

Consider a resource which has a polymorphic has-many relation .

Existing behavior: While creating the payload for the has-many relation should have the base class name as the type field in the relationships post data.

New behavior: Allows creating a resource which has a polymorphic has-many relation even with the subclass type.

Why: Because ember sends in the subclass modelName (the instance's modelName) while creating a resource of the said specifications.

I have raised a PR against the 0-9 branch as this is the stable branch and this version was working fine against our codebase (master had some errors which I did not spend time on investigating any further..)

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  Per https://github.com/cerebris/jsonapi-resources/blob/v0.9.3/lib/jsonapi/request_parser.rb#L489-L502, this issue is evident from the code, and I stumbled on it on my usecase.
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• Maintains compliance with JSON:API
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[st0012]

@saravanak thanks for the work! Are you going to fix the test failures though?

[lgebhardt]

@saravanak Thanks for working on this. Your changes have been pushed to release-0-9