Attributes on resources should be converted to symbol before storing in @_attributes

[hidde-jan]

Defining attribute "string_name" on a resource results to that string being used as the key for storing the attributes options in the @_attributes instance variable in https://github.com/cerebris/jsonapi-resources/blob/9f34830572094a7aa74fa7ccd91095fdc9fea529/lib/jsonapi/resource.rb#L519 (instead of a symbol).

This in turn leads to problems down the road, for instance in RequestParser, which converts incoming attributes from the params from the to symbols before passing it to resource_klass._attribute_options in https://github.com/cerebris/jsonapi-resources/blob/9f34830572094a7aa74fa7ccd91095fdc9fea529/lib/jsonapi/request_parser.rb#L573 leading to problems in https://github.com/cerebris/jsonapi-resources/blob/9f34830572094a7aa74fa7ccd91095fdc9fea529/lib/jsonapi/resource.rb#L754

Solution: convert attr to a symbol in https://github.com/cerebris/jsonapi-resources/blob/9f34830572094a7aa74fa7ccd91095fdc9fea529/lib/jsonapi/resource.rb#L509

This is a duplicate of #1019, but that issue is pretty vague w.r.t. what's the issue.