SPDY: fixed potential integer overflow while parsing headers.

SPDY: fixed potential integer overflow while parsing headers.

Previously r->header_size was used to store length for a part of
value that represents an individual already parsed HTTP header,
while r->header_end pointed to the end of the whole value.

Instead of storing length of a following name or value as pointer
to a potential end address (r->header_name_end and r->header_end)
that might be overflowed, now r->lowercase_index counter is used
to store remaining length of a following unparsed field.

It also fixes incorrect $body_bytes_sent value if a request is
closed while parsing of the request header.  Since r->header_size
is intended for counting header size, thus abusing it for header
parsing purpose was certainly a bad idea.