fix for iconv_mime_decode_headers function skipping headers

Summary: A malformed mime header causes hhvm to punt on the rest of the
decode. This diff is a port of the fix for https://bugs.php.net/bug.php?id=52941
since the code is almost identical and solves the problem with embedded '=' and
'?' in mime headers.

CVE-2010-4699

Reviewed By: @JoelMarcey

Differential Revision: D1689385