Fix bug where setting default for command-specific opt wasn't honoured

[jmglov]

For example, given a function with the following spec:

(defn process-file
  {:org.babashka/cli
   {:spec
    {:media-file
     {:desc "Media file to transribe"
      :ref "<file>"
      :require true}}}}
  [{:keys [media-file] :as opts}]
  media-file)

Calling cli/dispatch like so would result in process-file returning nil:

(cli/dispatch {:media-file "some-file.mp4"})

Please answer the following questions and leave the below in as part of your PR.

• [x] This PR corresponds to an issue with a clear problem statement: #70

• [ ] I have updated the CHANGELOG.md file with a description of the addressed issue. N/A: bugfix.

[borkdude]

Thanks dude!