RST writer: treat Span as transparent.

Previously an Emph inside a Span was being treated as
nested markup and ignored.  With this patch, the Span
is just ignored.

Closes #5446.

src/Text/Pandoc/Writers/RST.hs

@@ -472,6 +472,9 @@ flatten outer
           -- them and they will be readable and parsable
           (Quoted _ _, _)          -> keep f i
           (_, Quoted _ _)          -> keep f i
+          -- spans are not rendered using RST inlines, so we can keep them
+          (Span _ _, _)            -> keep f i
+          (_, Span _ _)            -> keep f i
           -- inlineToRST handles this case properly so it's safe to keep
           (Link _ _ _, Image _ _ _) -> keep f i
           -- parent inlines would prevent links from being correctly

test/command/5446.md

@@ -0,0 +1,6 @@
+```
+% pandoc -f latex -t rst
+{\em test test \/}
+^D
+*test test*
+```