Markdown writer: Ensure that + and - are escaped properly...

so they don't cause spurious lists.  Previously they were only
if succeeded by a space, not if they were at end of line.

Closes #3773.

src/Text/Pandoc/Writers/Markdown.hs

@@ -416,6 +416,9 @@ blockToMarkdown' opts (Plain inlines) = do
             '+':s:_ | not isPlain && isSpace s -> "\\" <> contents
             '*':s:_ | not isPlain && isSpace s -> "\\" <> contents
             '-':s:_ | not isPlain && isSpace s -> "\\" <> contents
+            '+':[]  | not isPlain -> "\\" <> contents
+            '*':[]  | not isPlain -> "\\" <> contents
+            '-':[]  | not isPlain -> "\\" <> contents
             '|':_ | (isEnabled Ext_line_blocks opts ||
                      isEnabled Ext_pipe_tables opts)
                     && isEnabled Ext_all_symbols_escapable opts

test/command/3773.md

@@ -0,0 +1,14 @@
+```
+% pandoc -t markdown
+A.  \#
+B.  \+
+C.  \*
+D.  o
+E.  o or \*
+^D
+A.  \#
+B.  \+
+C.  \*
+D.  o
+E.  o or \*
+```