HTML writer: stringify alt text.

Previously, if you had formatting in the label of an Image
element, you'd get escaped HTML in the alt attribute.  Now
you just get a plain string version, which seems preferable.

src/Tests/Writers/HTML.hs

@@ -38,4 +38,9 @@ tests = [ testGroup "inline code"
           , "nolanguage" =: codeWith ("",["nolanguage"],[]) ">>="
             =?> "<code class=\"nolanguage\">&gt;&gt;=</code>"
           ]
+        , testGroup "images"
+          [ "alt with formatting" =:
+            image "/url" "title" ("my " +++ emph "image")
+            =?> "<img src=\"/url\" title=\"title\" alt=\"my image\" />"
+          ]
         ]

src/Text/Pandoc/Writers/HTML.hs

@@ -602,8 +602,7 @@ inlineToHtml opts inline =
                                  if null tit then [] else [title tit]) $ 
                                  linkText
     (Image txt (s,tit)) -> do
-                        alternate <- inlineListToHtml opts txt
+                        let alternate' = stringify txt
-                        let alternate' = showHtmlFragment alternate
                         let attributes = [src s] ++
                                          (if null tit 
                                             then []