Markdown writer: Don't print title attribute if title is empty.

(This differs from the behavior of Markdown.pl, and agrees with PHP
Markdown.  But John Gruber has indicated that he prefers this behavior.)
Adjusted test suite accordingly.


git-svn-id: https://pandoc.googlecode.com/svn/trunk@1026 788f1e2b-df1e-0410-8736-df70ead52e1b

src/Text/Pandoc/Writers/HTML.hs

@@ -407,7 +407,10 @@ inlineToHtml opts inline =
     (Image txt (source,tit)) -> do
                         alternate <- inlineListToHtml opts txt
                         let alternate' = renderHtmlFragment alternate
-                        let attributes = [src source, title tit] ++ 
+                        let attributes = [src source] ++
+                                         (if null tit 
+                                            then [] 
+                                            else [title tit]) ++ 
                                          if null txt 
                                             then [] 
                                             else [alt alternate']

tests/writer.html

@@ -1033,7 +1033,7 @@ document.write('<a h'+'ref'+'="ma'+'ilto'+':'+e+'">'+'<code>'+e+'</code>'+'<\/'+
     ><img src="lalune.jpg" title="Voyage dans la Lune" alt="lalune"
        /></p
     ><p
-    >Here is a movie <img src="movie.jpg" title="" alt="movie"
+    >Here is a movie <img src="movie.jpg" alt="movie"
        /> icon.</p
     ><hr
      /><h1 id="footnotes"