Allow tag keyword class names combined with non-literal class names

Generate the correct class attribute of an element when the class names
are both present as:

- part of the element keyword (using the dot notation)
- a non-literal value of the :class key in the attribute map
  (specifically when using a symbol as the value).

Fixes issue weavejester#143

src/hiccup/compiler.clj

@@ -77,6 +77,7 @@
   (cond
     (nil? class)    classes
     (string? class) (str classes " " class)
+    (symbol? class) (list `str classes " " class)
     :else           (str classes " " (str/join " " class))))
 
 (defn- merge-attributes [map-attrs id classes]

test/hiccup/core_test.clj

@@ -110,7 +110,11 @@
     (let [times-called (atom 0)
           foo #(swap! times-called inc)]
       (html [:div (foo)])
-      (is (= @times-called 1)))))
+      (is (= @times-called 1))))
+  (testing "defer evaluation of non-literal class names when combined with tag classes"
+    (let [class-local-var "class-foo class-bar"]
+      (is  (= (html [:div.test {:class class-local-var}])
+              "<div class=\"test class-foo class-bar\"></div>")))))
 
 (deftest render-modes
   (testing "closed tag"