Clearing polymorphic relationship by sending null data with PATCH is broken again

从 lib/jsonapi/request_parser.rb 中的 parse_update_relationship_operation 开始，并将该回归与 issue #656 和 commit a232243f5e1e4a24cd9bb87406c5a229326e7ea2 进行比较。复现带有 polymorphic association 和 null data 的 PATCH relationship request；当 removal 不再引发 undefined-method error，且有 regression test 覆盖该情况时，即视为完成。

The original issue #656 seems to have regressed, as the same error is now raised for a polymorphic association removal.

Started PATCH "/tags/830926030/relationships/taggable" for 127.0.0.1 at 2017-07-05 11:33:31 +0300
Processing by TagsController#update_relationship as HTML
  Parameters: {"data"=>nil, "relationship"=>"taggable", "tag_id"=>"830926030"}
Internal Server Error: undefined method `[]' for nil:NilClass /Users/vesa/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonapi-resources-0.9.0/lib/jsonapi/request_parser.rb:594:in `parse_update_relationship_operation'
/Users/vesa/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonapi-resources-0.9.0/lib/jsonapi/request_parser.rb:133:in `parse_modify_relationship_action'
/Users/vesa/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonapi-resources-0.9.0/lib/jsonapi/request_parser.rb:98:in `setup_update_relationship_action'
/Users/vesa/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonapi-resources-0.9.0/lib/jsonapi/request_parser.rb:38:in `setup_action'
/Users/vesa/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonapi-resources-0.9.0/lib/jsonapi/request_parser.rb:27:in `initialize'
/Users/vesa/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonapi-resources-0.9.0/lib/jsonapi/acts_as_resource_controller.rb:66:in `new'
/Users/vesa/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonapi-resources-0.9.0/lib/jsonapi/acts_as_resource_controller.rb:66:in `process_request'
/Users/vesa/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/jsonapi-resources-0.9.0/lib/jsonapi/acts_as_resource_controller.rb:39:in `update_relationship'

The regression seems to stem from this commit a232243f5e1e4a24cd9bb87406c5a229326e7ea2 where operations were reworked to use an operation dispatcher and operation processors.

       if relationship.is_a?(JSONAPI::Relationship::ToOne)
         if relationship.polymorphic?
-          operation_args[1].merge!(
-            key_value: verified_params[:to_one].values[0] && verified_params[:to_one].values[0][:id],
-            key_type: verified_params[:to_one].values[0] && verified_params[:to_one].values[0][:type]
-          )
+          options[:key_value] = verified_params[:to_one].values[0][:id]
+          options[:key_type] = verified_params[:to_one].values[0][:type]

https://github.com/cerebris/jsonapi-resources/commit/a232243f5e1e4a24cd9bb87406c5a229326e7ea2#diff-a32d87f955d51d1e7270b1a974529f7eR585

I am not sure how to add tests to ensure this issue does not regress again. Any pointers on how you'd like to have such tests would be nice.

I discovered this issue as I'm adding special handling to jsonapi-authorization gem: https://github.com/venuu/jsonapi-authorization/pull/75