markdown writer: in a table cell ending with backslash, it's not escaped

I forgot whether escaping of literal backslashes is done at all in the writer: but it needs to be (or maybe the bug is only in tables). Otherwise the attached ascii table will load OK, but when writing it and re-loading it, the pipe character is treated as an escaped literal rather than the backslash being the literal, because it was written unescaped (/| instead of //| but I mean with backslashes... avoiding writing them here because of how jira treats them).