MediaWiki writer: don't softbreak lines inside list items.

Closes #3531.

test/command/3531.md

@@ -0,0 +1,19 @@
+```
+% pandoc -t mediawiki --wrap=preserve
+* This is a list item.
+* This is a list item in Markdown. It is
+  continued in the next line.
+    * It has a sub-item.
+* This is the next list item.
+
+A paragraph can span multiple lines
+without being broken into pieces.
+^D
+* This is a list item.
+* This is a list item in Markdown. It is continued in the next line.
+** It has a sub-item.
+* This is the next list item.
+
+A paragraph can span multiple lines
+without being broken into pieces.
+```