Change method used to retrieve rack status

[pareeohnos]

This is a very small change to the way jsonapi-resources fetches the status code from the symbol. Currently, it makes use of Rack::Utils::SYMBOL_TO_STATUS_CODE, however this has started to break on new versions of Rack.

A change introduced to Rack deprecates the status :unprocessable_entity in favour of :unprocessable_content. By using the Rack::Utils::SYMBOL_TO_STATUS_CODE hash directly, no status is found due to the internal changes, which results in jsonapi-resources returning a 0 status when there are validation errors.

This is the first step to fixing the issue and is fully backwards compatible. In the future however, the status :unprocessable_entity should stopped being used entirely, however this will require checks to see which version of Rack is being used. I haven't tackled this here as I wasn't sure how best you wanted to handle this but I'd be happy to take a look at it in the future.

This fixes the issue #1456

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