RST writer: strip whitespace within inlines, closes #4327 (#4329)

+ Whitespace is ignored at the beginning and end of emphasis, as per
  <http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#inline-markup>.
+ Export `stripLeadingTrailingSpace` from `Text.Pandoc.Writers.Shared`.

src/Text/Pandoc/Writers/RST.hs

@@ -379,7 +379,9 @@ blockListToRST = blockListToRST' False
 -- | Convert list of Pandoc inline elements to RST.
 inlineListToRST :: PandocMonad m => [Inline] -> RST m Doc
 inlineListToRST lst =
-  mapM inlineToRST (removeSpaceAfterDisplayMath $ insertBS lst) >>=
+  mapM inlineToRST ((stripLeadingTrailingSpace .
+                     removeSpaceAfterDisplayMath .
+                     insertBS) lst) >>=
     return . hcat
   where -- remove spaces after displaymath, as they screw up indentation:
         removeSpaceAfterDisplayMath (Math DisplayMath x : zs) =

src/Text/Pandoc/Writers/Shared.hs

@@ -41,6 +41,7 @@ module Text.Pandoc.Writers.Shared (
                      , unsmartify
                      , gridTable
                      , metaValueToInlines
+                     , stripLeadingTrailingSpace
                      )
 where
 import Control.Monad (zipWithM)

test/Tests/Writers/RST.hs

@@ -50,6 +50,15 @@ tests = [ testGroup "rubrics"
               , ""
               , "    quoted"]
           ]
+        , testGroup "spaces are stripped within inlines"
+          -- pandoc issue 4327 "The text within inline markup may not
+          -- begin or end with whitespace"
+          -- http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#inline-markup
+          [ "multiple" =:
+            strong (space <> str "text" <> space <> space) =?> "**text**"
+          , "single" =:
+            strong (space) =?> "****"
+          ]
         , testGroup "headings"
           [ "normal heading" =:
               header 1 (text "foo") =?>