Add tests for RpcController#update_feed
Bugfix: use Hash#merge! instead of Hash#merge

params.merge don't modify self so `link` in JSON data will be ignored, so Item.create raises ActiveRecord::StatementInvalid.
Stop modifying `params`, use copy
Accept @makimoto's review

https://github.com/fastladder/fastladder/pull/79#discussion_r3406321
Merge pull request #79 from fastladder/rpc_controller_update_feed_spec

Add test for RpcController#update_feed (and tiny bugfix)