Missing support for JSON serialization of "document" shapes

[GaspardP]

Hi. I found a similar issue solved in #232, but this time it's the serialization of data that's missing.

AWS has added a few shapes with the :document true field in the bedrock-runtime API (I haven't checked the other APIs for similar cases). For reference:

:ModelInputPayload {:type "structure", :members {}, :document true, :sensitive true}
:Document {:type "structure", :members {}, :document true}}

Here is an example for the expected behavior. Currently, the call to shape.json-serialize incorrectly returns "{}".

(deftest test-json-serialize
  (is (= "{\"this\":\"is\",\"a\":\"doc\"}"
         (shape/json-serialize {}
                               {:type "structure", :members {}, :document true}
                               {:this "is" :a "doc"}))))