Make po2html retain untranslated attributes

Without this change, the values of untranslated attributes were silently dropped. E.g., &lt;img src="URL" alt="English alt text" /&gt; was silently converted to &lt;img src="URL" alt="" /&gt; when the alt text was untranslated.

Tag contents were not affected by this problem, and the change does not seem to affect empty tags, e.g., &lt;title&gt;&lt;/title&gt;.

I am also fairly sure that I can enjoy another round of acronyms hitherto unknown to me, but I would rather spend the time improving my patch than reading Urban Dictionary, so if you find something lacking, please tell me so.

Guessing from the reaction to my last attempt to improve this project, I should probably write a unit test, but I am not exactly sure I know the project well enough to write one.