Correct handling of parents when creating a commit

I got the following exception and stack trace while attempting to create a commit using tentacles:

    com.fasterxml.jackson.core.JsonGenerationException: Cannot JSON encode object of class: class clojure.core$parents: clojure.core$parents@5414c36d
        at cheshire.generate$generate.invoke(generate.clj:147)
        at cheshire.generate$generate.invoke(generate.clj:119)
        at cheshire.core$generate_string.invoke(core.clj:32)
        at cheshire.core$generate_string.invoke(core.clj:19)
        at tentacles.core$make_request.invoke(core.clj:81)
        at tentacles.core$api_call.invoke(core.clj:91)
        at tentacles.data$create_commit.invoke(data.clj:46)

We're trying to encode the Clojure `parents` function when we actually need to leave the `:parents` key alone in the options (since the user will have already provided it).