defcard-rg without a name causes compilation error

Minimal code to reproduce:

```clojure
(defn hello []
[:div "hello"])

(defcard-rg
[hello])

;; [Figwheel:SEVERE] java.lang.ClassCastException: clojure.lang.Cons cannot be cast to clojure.lang.Associative

```

I suspect that the problem is related to `devcards.core/merge-options` and macro expansion.

Start with the minimal Clojure example in the issue and inspect devcards.core/merge-options during macro expansion. Reproduce the ClassCastException for defcard-rg without a name, then verify that the same form compiles without the error after the fix.

Written by the indexing model from the issue text.