Prefix even empty figure names with "fig:"

The convention used by pandoc for figures is to mark them by prefixing
the name with "fig:".  The org reader failed to do this if a figure had
no name.  The test for this was broken as well.

This fixes #2643.

src/Text/Pandoc/Readers/Org.hs

@@ -2,7 +2,7 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses, FlexibleContexts, FlexibleInstances #-}
 {-
-Copyright (C) 2014-2015 Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
+Copyright (C) 2014-2016 Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
 
 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
@@ -21,7 +21,7 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 
 {- |
    Module      : Text.Pandoc.Readers.Org
-   Copyright   : Copyright (C) 2014-2015 Albert Krewinkel
+   Copyright   : Copyright (C) 2014-2016 Albert Krewinkel
    License     : GNU GPL, version 2 or above
 
    Maintainer  : Albert Krewinkel <tarleb+pandoc@moltkeplatz.de>
@@ -628,7 +628,7 @@ figure = try $ do
          maybeNam <- lookupBlockAttribute "name"
          guard $ isJust maybeCap || isJust maybeNam
          return ( fromMaybe mempty maybeCap
-                , maybe mempty withFigPrefix maybeNam )
+                , withFigPrefix $ fromMaybe mempty maybeNam )
    withFigPrefix cs =
        if "fig:" `isPrefixOf` cs
        then cs

tests/Tests/Readers/Org.hs

@@ -635,11 +635,11 @@ tests =
                   ] =?>
           para (image "edward.jpg" "fig:goodguy" "A very courageous man.")
 
-      , "Unnamed figure" =:
-          unlines [ "#+caption: A great whistleblower."
-                  , "[[snowden.png]]"
+      , "Figure with no name" =:
+          unlines [ "#+caption: I've been through the desert on this"
+                  , "[[horse.png]]"
                   ] =?>
-          para (image "snowden.png" "" "A great whistleblower.")
+          para (image "horse.png" "fig:" "I've been through the desert on this")
 
       , "Figure with `fig:` prefix in name" =:
           unlines [ "#+caption: Used as a metapher in evolutionary biology."