DocBook reader: Better handle elements inside code environments.

Of course, we can't include structure in the code block, but
this way we at least preserve the text.  Closes #1449.

src/Text/Pandoc/Readers/DocBook.hs

@@ -769,7 +769,12 @@ parseBlock (Elem e) =
                                 ""   -> []
                                 x    -> [x]
            return $ codeBlockWith (attrValue "id" e, classes', [])
-                  $ trimNl $ strContent e
+                  $ trimNl $ strContentRecursive e
+         strContentRecursive = strContent . (\e' -> e'{ elContent =
+                                               map elementToStr $ elContent e' })
+         elementToStr :: Content -> Content
+         elementToStr (Elem e') = Text $ CData CDataText (strContentRecursive e') Nothing
+         elementToStr x = x
          parseBlockquote = do
             attrib <- case filterChild (named "attribution") e of
                              Nothing  -> return mempty

tests/docbook-reader.docbook

@@ -89,6 +89,9 @@ sub status {
     print "working";
 }
 </programlisting>
+    <screen>
+% <command>ls</command>
+</screen>
     <para>
       A list:
     </para>

tests/docbook-reader.native

@@ -23,6 +23,7 @@ Pandoc (Meta {unMeta = fromList [("author",MetaList [MetaInlines [Str "John",Spa
 ,BlockQuote
  [Para [Str "Code",Space,Str "in",Space,Str "a",Space,Str "block",Space,Str "quote:"]
  ,CodeBlock ("",[],[]) "sub status {\n    print \"working\";\n}"
+ ,CodeBlock ("",[],[]) "% ls"
  ,Para [Str "A",Space,Str "list:"]
  ,OrderedList (1,Decimal,DefaultDelim)
   [[Para [Str "item",Space,Str "one"]]