Fix @ code blocks

In cases where we had some horizontal space before the closing ‘@’, the
parser would not accept the block as a code block and we'd get ugly
output.

src/Haddock/Parser.hs

@@ -365,6 +365,7 @@ codeblock d =
       where
         p isNewline c
           | isNewline && c == '@' = Nothing
+          | isNewline && isSpace c = Just isNewline
           | otherwise = Just $ c == '\n'
 
 hyperlink :: Parser (Doc a)

test/Haddock/ParserSpec.hs

@@ -410,6 +410,14 @@ spec = before initStaticOpts $ do
           , "@"
           ] `shouldParseTo` DocCodeBlock "foo\n@\nbar\n"
 
+      it "accepts horizontal space before the @" $ do
+        unlines [ " @"
+                , " foo"
+                , ""
+                , " bar"
+                , " @"
+                ] `shouldParseTo` DocCodeBlock " foo\n\n bar\n "
+
       it "accepts unicode" $ do
         "@foo 灼眼のシャナ bar@" `shouldParseTo` DocCodeBlock "foo 灼眼のシャナ bar"