Fix HTML_ATTR rendering (naive)

- was missing spaces between attributes when rendering to org
- was duplicating attributes when rendering to html - now we join / replace
  attributes depending on the name - for now only class & style are appended

org/testdata/keywords.html

@@ -1,5 +1,5 @@
 <figure>
-<div class="highlight" class="a b c" id="it">
+<div class="highlight a b c" id="it">
 <pre>echo &#34;a bash source block with custom html attributes&#34;
 </pre>
 </div>