Escaped pipes in table headers are handled incorrectly

[jasharpe]

The following markdown is not rendered correctly:

a | \| | c
--- | --- | ---
a | b | c

I would expect this to render as a table with a literal "|" in the top middle cell, but instead this isn't rendered as a table at all (the escaped pipe breaks table parsing). Escaped pipes work find in the table body - they're only broken in the header.