Preserve mw:Placeholder separators

Fixes test cases like
{|
|foo

|-
|hello
|}

Also fixes the 'Table with row followed by newlines and table heading' wt2wt
test case, so this is somewhat covered by parser tests.

Also fixes newline preservation in the new parser test added in
0d45f2075d691c6e4d859fdf1af7b6ec89043022.

Change-Id: Idcbfd77fc5be3d9125cb59307599e6aac42ed137