RST writer: fix #7039.

We were losing content from inside spans with a class,
due to logic that is meant to avoid nested inline
structures that can't be represented in RST.

The logic was a bit stricter than necessary.  This
commit fixes the issue.

src/Text/Pandoc/Writers/RST.hs

@@ -519,8 +519,8 @@ flatten outer
           (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
+          (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