LaTeX(Beamer) writer: include contents under headers < slidelevel.

Currently we keep the fancy title slide and add a new
slide with the same title and whatever content was under the
header.

This changes behavior of slides, but is consistent with the
new behavior of the revealjs and other HTML slide show writers.

See #4317.

src/Text/Pandoc/Writers/LaTeX.hs

@@ -448,7 +448,14 @@ elementToBeamer slideLevel  (Sec lvl _num (ident,classes,kvs) tit elts)
                     : tit ++ [RawInline "latex" "}"] )
              : bs ++ [RawBlock "latex" "\\end{block}"]
   | lvl <  slideLevel = do
-      bs <- concat `fmap` mapM (elementToBeamer slideLevel) elts
+      let isSec Sec{} = True
+          isSec _     = False
+      let (contentElts, secElts) = break isSec elts
+      let elts' = if null contentElts
+                     then secElts
+                     else Sec slideLevel [] nullAttr tit contentElts :
+                          secElts
+      bs <- concat `fmap` mapM (elementToBeamer slideLevel) elts'
       return $ Header lvl (ident,classes,kvs) tit : bs
   | otherwise = do -- lvl == slideLevel
       -- note: [fragile] is required or verbatim breaks