Add test for transform java.lang.ClassCastException

When `obj` argument is a string (X)HTML payload and
not a string URL or URI, the following exception is
thrown:

```clojure
java.lang.ClassCastException: class java.lang.Class cannot be cast to class clojure.lang.IFn
```

test/html_to_md/transformer_test.clj

@@ -0,0 +1,13 @@
+(ns html-to-md.transformer-test
+  (:require
+   [clojure.string :as str]
+   [clojure.test :as t :refer [deftest is testing]]
+   [html-to-md.html-to-md :refer [markdown-dispatcher]]
+   [html-to-md.transformer :refer [transform]]))
+
+(deftest transform-payload
+  (testing "String `obj` for: 3. A string representation of an (X)HTML fragment;"
+    (is (= "# This is a header"
+           (str/trim (-> "<h1>This is a header"
+                         (transform markdown-dispatcher)
+                         (first)))))))